For an ability that instakills the unit directly in front of it when used, I'd use a set HP with !ENEMY, 0. That should instantly kill the unit. For dramatic effect, you can look up a sound directory at https://maurits.tv/data/garrysmod/wiki/wiki.garrysmod.com/index8f77.html, as the hl2 sounds are ported to GMod. Hope this helps!