Although I don't know the triggers for the warmonger trait, they probably are dependent on the primary personality traits. An energetic, selfish character is more likely to enjoy war than an indolent but selfless one. Also, remember that you can only see the broad trends of these traits on the character scroll. There are actually six levels for each of the six personality traits.
Also, locker-room talk aside, most people have strong inhibitions against killing another human being. I can't give you a source, but I read that research from the WW2 showed that, in an average unit, most enemy casualties were the work of just 10% of the men. They were the ones that were actively trying to kill their opponents. The remaining 90% would kill when they or their mates were under direct threat, but otherwise fire blindly and avoid lethal confrontation.
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