Read an article about psychopaths and work scanning their brains.
It seems to show that unfortunately for them and their victims, they are (physically) "wrong in the head" and suggests they need treatment rather than punishment. Not that it's clear what the treatment should be.
I fell to thinking how lucky I am. I don't endure great moral struggles to avoid being a psychopath - I just wasn't born to it. Similarly, I don't "battle" with alcoholism, tobacco, gambling, weight or any of the many other "unacceptable" things. The last time I got legless-drunk was about 6 years ago at a festival where there wasn't a dance to go to and we sat around savouring one nice bottle after another. Keeping sober (etc.) isn't difficult for me, I simply have "lucky" biology.
I'm not arguing that mass-murderers should be allowed to wander around as if nothing had happened but executing them (as happens in the US) is going way too far. If, as may be the case, it's genetic, future generations may dust off the "genocide" label.