Animal Mutilation

If you actually kill someone, you did something that's not that easy to do. The reason abused women typically do more time is they typically plan it and pick up an equalizer.

Aside from things like vehicular manslaughter, killing someone is not easy. It's not generally "OOPSIE!" and it's unlikely that without preparation of some kind, you kill someone in self defense first try.

Serial killers are known to start with mutilating and killing animals as children. This is probably preparation -- learning how to kill.

It's also likely psyching themselves up intentionally.

Child neglect and abuse is rampant. I'm reluctant to believe "some people are just born bad." 

Probably in most cases these are abused children expressing their anger in a fashion that is preparing them to protect themselves from someone with lethal force. And having done it once or twice in self defense, somehow this leads to a killing spree that never ends.

Rather than assume these are bad kids, when animal mutilation by a child is discovered, perhaps law enforcement and social workers and the like should wonder who is abusing this kid so badly that a child is taking steps to prepare themselves to fight back.

Perhaps addressing it that way would be more effective at preventing bad things than vilifying the children.

It's probably not an easy thing to fix even at that stage, but once you have a serial killer, we probably trend towards warehousing them for decades without hope of rehabilitation.

Footnote 

It would be interesting to do a study and determine if nutcase serial killers are people who successfully killed their actual abusers or if they killed someone else as a form of emotional outlet because they never succeeded in meaningfully protecting themselves from their abusers and then this became a pattern. An extreme form of "kicking the dog" the rest of your life because you feel helpless.