What do we do while deciding if free will exists?

[AFTT47]

I would especially like to see determinism applied to our so-called justice system. It's largely based on punishment/revenge and is IMO, unenlightened and outdated. It should be based on restitution to the victim, rehabilitation and when necessary, removing dangerous people from society until they are deemed safe. There needs to be deterrence too but that shouldn't take the form of tossing a non-violent person in a prison cell and deliberately making them miserable. Where's the benefit in that?