Letting him out definitely won't benefit society. The recidivism for "mentally insane" convicts is extremely high.
It feels good, but it's like letting a drunk driver get back behind the wheel after they show remorse. The chances that they get in another DWI (and hurt someone) is high.
It's also why we have sex offender registries. The recidivism is high.
I'd love to read the papers you've consumed that lead you to this consultation if you have them handy. A lot of what we see in Canada is in fact the exact opposite in the context of mental health, so I'm very curious to know where you formed this opinion from.
And also it's because he's still a person. A person who did a terrible thing while literally insane, but still a person.