I don't think that's an area known very well even in RL. Some people are so delicate they can't even bear to see blood. Some can slay non-mammals no problem, but would feel uneasy if required to kill for example a cute bunny. Others can kill any animal without a second thought, yet would suffer mentally if they killed a fellow human in cold blood. Lastly there are those for whom human lives are inconsequential. However, I reckon it's compeletely unknown how much of this is due to anything genetic, how much to the surroundings, apart from obvious mental disorder cases. So, I'd say that no matter how much they were genetically manipulated, they still need thorough training and psychological conditioning.