Well, there are different kinds of killers and a normal person is just as capable of killing as an abnormal person. Self defence for instance.
I,personally would find it extremely difficult to kill someone,even in self-defence,so I don't know why my own brain functions that way. I'd put up a fight because a coward I'm not,but so far as the thought of killing,I couldn't do it.
However,those in the forces have to kill,but the effects of their self-defence lasts a lifetime and duly affects their brains in a different way from the cold-blooded killers.