I'm sure they have a fully functional brain. They know the score and know what will happen.
They don't do it for fun.
I meant that their brain is measurably different from ours. You can't understand how a normal person would do this. I can understand how an abnormal person would do this. Something significant changed in this person's brain, or it was never normal to begin with. Once again, it doesn't excuse the behavior, it's just kind of easy to comprehend how this happens.
A high percentage of serial killers had major brain injuries as children, particularly to the frontal lobe. Many child molesters went through major trauma as children as well. Trauma can change how your brain works. My point is just that it's not a normal dude just randomly having the urge to murder a bunch of people. It's an abnormal brain that causes things like this. I'm just responding to your initial comments saying that you don't understand these things happening, and that you would do something different. Of course you would, because you don't have a broken brain.