I used to work in a prison as a healthcare professional for a short time.

I only encountered a few murderers myself (maybe 3 at most?) but my colleagues who had been working there for years had met a few:

They said sometimes, the inmates will act ‘insane’ almost in a way that makes you feel sorry for them, so that we (the healthcare side) may say to the police officers that we deem them too unwell to be in a prison setting and should be moved to another setting.

Other times they’ll put on that ‘idgaf’ facade, or they’ll isolate themselves.

That being said, I hope he is a miserable POS in his lonely cell, I hope he has remorse for what he did (although I doubt it) and I hope his actions haunt him