Just thought I'd add the actual wordings from the two books
Killer: You made your first kill at a very young age and found the task of war or murder to your liking. You either take particular pride in a well-placed blow, or find vile pleasure in such a strike as you twist the blade to maximize the pain. You deal additional damage equal to your weapon's critical hit modifier when you score a successful critical hit with a weapon; this additional damage is added to the final total, and is not multiplied by the critical hit multiple itself. This extra damage is a trait bonus.
Brute: You are an especially large and hulking individual, and when you strike, you aim to deliver the maximum amount of pain. Whenever you score a critical threat, you add (but not multiply, in the case of a confirmed critical hit) an amount of damage equal to your weapon's critical modifier.
My confusion stems from the fact that one of these damage bonuses is independent of the other, but they use the same source as the bonus damage. Any thoughts on this would help alot.