Revenge will not bring them back or give any closure, but a swift, lawfully painful punishment makes an example. That's what the law is for. To separate dangerous people from the society they would bring harm to, and to send the message that illegal activity will be punished. Someone may try to kill and or rape him, or he could get into some sort of hierarchy and become the boss of a prison clique. There is a LOT wrong with our judicial system that I don't want to go on about, but violent offenders like this guy should get what's coming to them. We're lucky. He turned himself in when he was caught. He could have killed a bunch of cops, escaped, and there could have been a man hunt for him. Or worse... we could never find out who it was, and he could have gotten off Scott free. It happens.If he is found not to be insane, and does get sentenced to prison, he's going to be such a big target. Just like with Jeffrey Dahmer, somebody in prison will want to be famous for wasting him.
I'm sure a lot of people in this country would love to exact some kind of revenge on him. But no matter what his fate may be, it won't bring the twelve victims back, nor will it bring closure to the victims' families any faster.
But the fact of the matter is, an insane person COULD very well set up a trap like that. Insane doesn't mean you're blinded by crazy and you poo yourself for fun. it means that something is dangerously wrong with you. He probably hid it quite well (even though NO ONE mentioned the disturbing messages he left about the gun range until it was too late). Crazy certainly does not mean that he wasn't cold and calculating. Those who have the frightening vision to pull off what he pulled off surely can't be in the right mental state.
Though something tells me, interviews with people are going to piece together obvious flaming red flags that could have prevented all of this. But that's what always happens in the face of a tragedy.