Compulsion? Yes. You wonder what pushed them over the line. Some don't need much of a push, and others wouldn't dream of hurting a fly, but when circumstances are right, they close their eyes and pull the trigger. And then what happens to them mentally? Was the victim guilty, too? Oh, there are endless questions.
Right. There are always questions about the victim. We say no one deserves to be murdered....but there are cases where the "victims" were abusers. There are psychopaths who do seem to have a compulsion to kill, in order to assimilate power or some kind of identity they are lacking. And then the seemingly ordinary people who are pushed over the edge some how. And it's those questions that writers and readers of mystery pursue....:)