Serial killers in general are perfectly aware of what they are doing, they just don't care about the judgement imposed by society on such actions and at the same time the personal payoff is too great to ignore.
What we should be concerned with is how we as a society handle mental health as a medical condition, not as a legal defense. I am talking about early detection, treatment, and if necessary preemptive detainment, not about how we handle the people we who have already gone Rambo.