they should rehabilitate them
The fact is that usually serial killers and child murderers are usually not aggravated to kill. There is no point in rehabilitating them. Even if they are rehabilitated, the horrendous crime which is worse than a single murder will still be on their record and they will be screwed for life.
Besides, do you know how much money we would waste trying to rehabilitate every wacko who murders kids and multiple people for no reason? Why would society want them back?