I can give one reason. Its not logical but it’s a reason. Its to signal don’t fuck with me because look what happened to this last guy. If someone in your day to day life messes with you and you allow it, are you not inviting more of that behavior. If you punish them for their behavior they will be less likely to do it again. I think its some deep seeded feeling of justice that someone who fucks around receives punishment. Wanting to rehab someone who did something cruel and beyond reason like murder isn’t something I have an instinct for. I would argue its the opposite of human nature to want it. If someone murders my daughter. and then ends up using my tax dollars to sit in a facility where they’re given respect and care and amenities to enjoy and be comfortable I wouldn’t feel like justice was served as the state has taking away my ability to dish out the most basic urge we have to make sure the people in society who act criminally are handled. The state should consider punishment as a form of sentencing because it is a form of justice.