The Corruption bar governs both available wargear and abilities in game, but also the game ending.
There's a total of 5 different endings, but honestly I only think 3 of them are worth the playthrough. Those endings are unlocked by playing

• Full Purity (no squads above 4 Corruption when campaign ends)
  
• Purity after Treachery (One squad at or above 4 Corruption for The Wages of Sin)
  
• Full Corruption (Succumb to Chaos in its entirety)

Who the Traitor is is decided just before the "The Wages of Sin" mission.
Whichever squad has the highest Corruption rating will be chosen as the traitor. If multiple Squads have the same corruption rating, the character with the lowest Will Discipline is chosen (I think).

NOTE: Davian Thule is incorruptible
NOTE: The Force Commander can never be chosen as the Traitor.

This path aims to retain Purity for all your squads, meaning that no squad can go above 4 Corruption points.

• Access to powerful "Holy" Wargear
  
• Some Purity Traits (Namely from Avitus, Cyrus, and Jonah) are rather useful
  
• Davian Thule (Dreadnought) stays with you throughout the campaign
  
• Powerful Supremacy-Synergy Traits (which become useless when Squads are fully corrupted)
  
• You lose no Squad members to treachery
  
• FOR TEH EMPRAH!

• You lose access to powerful (Corrupting) Wargear
  
• You must be careful not to fail (too many) Corruption objectives (don't save Guardsmen/Scouts in time, spend too long aboard Judgement of Carrion, etc.)
  
• You are forced to take certain squads on certain missions
  
• In lieu of a Traitor, you have to fight Martellus in his super-swole Chaos Predator. Which ♥♥♥♥♥♥♥ sucks, as it is easily one of the hardest encounters in the game - unless you cheese it.

The Full Purity playthrough is, arguably, the hardest way to complete the game (somewhat alleviated by the imbalance of Ranged Damage+Synergies)
While some items like Oath of Penance can help dodge corruption points that you fail to avoid (Saving all Scouts on Calderis is one of the trickier objectives to fufill), you must generally play carefully.
You also need to bring certain squads to certain missions, to avoid Corruption (Cyrus to "The Rescue", Thaddeus to Spire Legis, Tarkus against the Eldar avatar, Jonah Orion to recover relics on the Judgement of Carrion).













FORCE COMMANDER
AVITUS
TARKUS

With the trio of Supremacy-Synergy (Avitus Battle Supremacy, FC Command Supremacy, Tarkus Tactical Supremacy) your 3-squad setup becomes an unstopable heretic-♥♥♥♥♥♥♥, ridiculousness. Pop Tactical Advance, Focus Fire, and let loose a Battle Cry then quietly sip your imperial tea as heretics, ork hordes, tanks, Eldar War Gods, and daemons melt before your attack-move ♥♥♥♥-squad.
If you're still confused, let me spell it out for you.
Maxing out the energy tree gets you

• TARKUS: Tactical Expertise; making Tactical Advance affect nearby allies. Tactical Supremacy Tactical Advance gains bonuses when combined with Command Supremacy and Battle Supremacy.
  
• AVITUS: Focus Mastery making Focus Fire benefit nearby allies. Battle Supremacy Focus Fire grants bonus range, and gives bonuses when combined with Command Supremacy and Tactical Supremacy.
  
• FC Command Supremacy making War Cry reduce the Energy cost of abilities for nearby allies. and gives special benefits when combined with Tactical Supremacy and Battle Supremacy.

Result?
Your entire force will...

• Take reduced ranged damage
  
• Be immune to suppression
  
• Be immune to knockback
  
• Have Increased melee damage
  
• Have increased ranged damage
  
• Have increased weapon range
  
• Have a chance to instant-gib enemy infantry on hit
  
• Have reduced energy-costs with every use of War Cry
  (yes, EVERY USE, and it STACKS. Meaning 250-energy signums go down to 7 or 0 energy per use towards the end of deployments)

Note: With this setup you should use a bolter (Terminator Armor: Storm Bolter) on Tarkus and the FC (alternatively a Plasma gun on the Force Commander).
Note: As you may have noticed, I only pointed out 3 Heroes. The fourth is a flex slot, and I recommend Thule, Cyrus (EXPLOSIVES!) or Jonah Orion. See the "Generic Builds" section

Additional Tips:
-In the far upper right corner of the Judgement of Carrion, during the "Keys of the Past" mission, you can find a crate holding a holy suit of terminator armor.

-The Force Commander's "Orbital Command" Trait allows him two uses of Orbital Bombardment - which is usually enough to cheese otherwisely difficult encounters, such as Martellus' Chaos Predator.

This path aims to retain Purity for all but a single squad, meaning that only one squad goes above 4 Corruption points.

• Access to powerful "Holy" Wargear
  
• Some Purity Traits (Namely from Avitus, Cyrus, and Jonah) are rather useful
  
• Davian Thule (Dreadnought) stays with you throughout the campaign
  
• Powerful Supremacy-Synergy Traits (which become useless when Squads are fully corrupted)
  
• You gain your choice of powerful wargear from defeating the traitor you designated
  
• FOR TEH EMPRAH!

• You lose access to powerful (Corrupting) Wargear
  
• You must be careful not to fail (too many) Corruption objectives (don't save Guardsmen/Scouts in time, spend too long aboard Judgement of Carrion, etc.)
  
• You are forced to take certain squads on certain missions

The Partial Purity playthrough is hard, but not as hard as Full Purity. Complete all Redemption objectives, and try to minimize Corrupting effects.
With this setup you retain purity for all but a single squad, letting the remaining squad become the traitor (4+ Corruption points). This also allows you control of the wargear you gain from completing "The Wages of the Sin" - where you defeat your choice of traitor.

An easy way to designate a traitor is to not bring him on his personal mission (leave Cyrus home from "The Rescue", don't bring Thaddeus to Spire Legis, don't bring Tarkus vs. the Eldar, or deny Jonah when recovering relics from The Judgement of Carrion.
BE CAREFUL to not accidentally redeem your designated traitor, by bringing them on a later redeeming mission.

Alternatively, you can use Corrupting powers for a single squad and simply lose them when you get to the Wages of Sin. Don't do this with Tarkus/Avitus though... Don't be an idiot.


-See "Full Purity" playthrough, as it is almost identical in style.

Additional Tips:
-See "Full Purity" playthrough, as it is almost identical in style.

This path aims to unlock all the Corruption Traits of your squads, while using the most powerful Corrupting Wargear.

• Acces to some of the most powerful wargear in game (including the Blighted Bolter and Power Armor for Tarkus)
  
• Can safely ignore Redemption missions; no need to juggle Corruption points
  
• Access to ridicuously overpowered Global Abilities
  Notably the Force Commander's "Unholy Revival" will get you out of the worst ♥♥♥♥, care free.
  
• Tarkus becomes a freaking Plague Marine God

• Lose access to Purity Traits (there are only two or three actually useful traits though)
  
• Captain Thule aien't having none of that, and won't be available after you reach a certain Corruption threshold
  
• Lose access to select pieces of "holy" Wargear, which cannot be used by the Corrupt
  
• HERESY

This path is quite simple, and to be honest, fairly easy to play. Use all the Corrupting wargear you find (which is conviniently more powerful than most regular wargear), "complete" all Corruption objectives (which usually just means: no need to be fast or efficient), use the Global Abilities as often as you can (as they add Corruption with each use). Enjoy Tarkus becoming the most broken Hero in the game, as he steamrolls entire armies with his epic Blighted Bolter and Power Armor.

To be honest, a lot of things go with this, as Corruption offers a lot of leeway. But with that being said, a few points must be stressed; Firstly Tarkus is the only member of your team that can use the incredibly powerful Blighted Bolter, so making him a melee-oriented squad is a waste of potential.
It is also important to notice that Corruption will "morph" certain abilities. More specifically:

• Tarkus' Tactical Advance becomes Plague Aura
  
• Tarkus' Taunt (if he has it) becomes Aura of Rage
  
• Avitus' Focus Fire becomes Fire Frenzy
  
• Force Commander's Battle Cry becomes Call for Blood

Due to these changes, the Energy-Line becomes much less powerful, as the ultimate Supremacy Traits (Command, Tactical, and Battle Supremacies) become useless - as the abilities they affect are replaced with chaotic versions.
With that being said; The Energy-Line still has useful traits for a lot of Heroes, but the Supremacy Traits are a waste of points.


NOTE: The "Legis Uprising" and "The Rescue" missions have Redemption objectives that, ironically, provide Corrupt wargear - and it's pretty good corrupt wargear, at that.

With this build Cyrus becomes a Vehicle-busting, Fortification-smashing, infinitely-invisible, super medic. Really; he's practically all you need to complete the campaign.
"Mines" becomes "Cluster Mines" that are thrown in bunches of 5, allowing you to ♥♥♥♥ up vehicles and large bunches of infantry, costing you nothing but energy.
Demolition Packs and Remote Detonators become energy-base, allowing you infinite army/building-wiping explosives for each deployment (just make sure you don't run out of Energy and exit stealth in the middle of an enemy army).
Furthermore; the Instant Revival and Silent Revival Traits allows Cyrus to restore downed squad leaders without ever puting himself at risk.

EXTRA TIPS:

• Jonah's "Unholy Vigor" corruption ability will fully restore the energy of all squads - allowing you to replenish Cyrus' energy even when he's miles away from fellow space marines
  
• If Cyrus gains enough corruption he unlocks the "Ethereal" trait, making him imune to damage so long as he's stealthed. This means you don't even have to worry about detonations going off "danger-close" when he's out and about.
  
• With the Force Commander's "Command Supremacy" trait, you can reduce the cost of Cyrus' abilities (excepting Stealth) to null. Meaning that yes; you can quite litterally, and quite easily, solo entire missions with Cyrus alone.
  
• With Full Purity, Cyrus gains "Mend" - allowing him to use Stimulant Kits with energy, and revive allies at full health. This is particularly powerful with Captain Thule and The Force Commander, as they do not need to rely on squad reinforcements to reach full health.

Possibly the only 'squad' that benefits from a hybrid build.
Spec into Energy to unlock Venerable and Machine Duel - allowing him to infuse your squads with awesome combat bonuses (Space Marines do love the sight of a Venerable Dreadnought smashing xenos filth).
Spec into Ranged Discipline and equip a Multi-melta to turn enemy vehicles into a steel slurry.
Invest in melee to turn Thule into a one-punch-man who turns whole armies into mush with a single use of his Emperor's Fist'ing.

I personally like to max stamina and energy, keeping him as a "heal-bot" with Veil of Time up - which accelerates movement- and attack-speed of your units, allowing them to speed across the battlefield in a hurricane of bullets.
You can also go into ranged discipline (equipping Smite and Ignite Soul) to have him spam flames and lightning like a warlock on crack.
Alternatively; Use Empower + Avitus' Signum Barrage (with Artillery Specialist, to make it energy based), and laugh away as you spam low-cooldown, no-mana cost Signum Barrages to shower an area with a torrential downpour of grenades.

More a gimmick than a useful build, as this sadly makes Tacical Supremacy hard to utilize and wastes Tarkus' formidable bolter abilities.
But with a few points in Stamina, and further into Strength; Taunt becomes a melee Challenge; causing all nearby enemies to engage Tarkus in melee combat. This makes otherwisely dangerous foes like Wraith Guard and setup-teams charge uselessly into combat like retarded slugga boys.