The "serial killers never stop" is a myth. In fact it's pretty common. Life happens to even the worst people, and they are unable to carry out their previous hobbies... In this car being murder.
Once you cross the bridge for "your thing", it feels to good to stop. Be it murder, rape or sex with minors. This is actually one of the reasons why it is so important to catch a criminal. Not to punish for previous crimes but to prevent new ones.