It's a vague question because I was trying to probe what people would find acceptable and unacceptable. If it helps, think of several examples maybe... One possibility would be altering the mechanics of aggression in their brains so that it only triggers in self-defense, or the use of pacification gas, or whatever. A variety of possible parameters. If this makes the question too general for you to answer, well, I didn't have a precise scenario in mind.