So, as fun as the idea is, in vanilla, this civic sucks:
-It forces you to be either militarist + xenophobe or authoritarian, because you have to have slaves. That means no fanatic spiritualist raiders, no militarist technocracy, you're pigeonholed into a very limited set of ethics, to a far larger degree than most civics
-It offers no bonuses that you can't get with one AP
-Despoilation, while again a really fun idea, doesn't really give you that much, especially in the early game, and by the time we get to late game you could just get the AP.

This excellent mod goes some way to rebalance it, by adding some bonuses to slaves and workers, but it doesn't address the core problem of pigeonholing barbaric despoilers. What if you wanted to play a mandalorian style empire, militaristic egalitarian raiders accepting all aliens? No can do. What about being like those marauders in-game, fanatic spiritualist raiders? again, no can do.

I believe one way we could make this civic more interesting and flexible is by removing the non-militarist ethics restrictions, and adding something akin to a resource and happiness boost while at war. Stealing pops shouldn't have to equate to enslaving pops. Your empire should be whatever you want it to be, and that might be a race of honourable raiders who select the worthiest among their prey to join their ranks, or anything else. This way you could still have your slaves as a xenophobic or authoritarian empire, or you could have a xenophilic, spiritualist, egalitarian or materialistic empire if you fancy.
The wartime bonus to resource production and happiness is pretty self-explanatory. Now the only thing this civic forces is a militaristic empire, adding a war-themed bonus is only appropriate, and what better way to show the joys of carnage and spoils of war than, say, a 10% bonus to resource production and happiness while at war?