You're misunderstanding. You can hold that the killer isn't 'morally' responsible, ie: deserves to be tortured for their crime, but still imprison them indefinitely for the good of everyone else.
This is more or less already the case in the western judicial systems, where punishment is not meant to be a revenge. The harsher punishments (like executions) are meant to deter.
I think there are exceptions like Nuremberg trials where the executions were more of a revenge.
