It sounds like you are arguing that the third group does not exist. If so, I think you are wrong about that.
It sounds like you are arguing that the third group does exist. If so, I think you are wrong about that. In the context of mass shootings the are almost always murder suicides or the individual is so insane they are either not capable of or at least not applying that kind of logic to their actions, like the Aurora CO. shooter.
Its not like we are talking about embezzlement, speeding, drug use, or even armed robbery here. There is little to suggest these mass murders give any thought at all to a future past their attack. The exception seemingly when they plan to die in the attack and become infamous. I for one do not fear or being prosecuted for possession of illegal fire arms, any other sort of weapon, let alone murder has any impact on their actions no would it no matter what you make the penalties.
If anything the threat of being purged from existence might be best. You and your property will be burned, people will be asked no to speak of you by name, birth records destroyed, any remains that speak to you ever having existing will be buried deep in an unmarked undiscovered location. That might give some of them pause.