Why is humane "under-punishment always preferable to over-punishment" top priority? You can construct a mathematically absurd example - e.g. if we under-punish a psychopathic attempted murderer and he gets an opportunity to actually murder, we've done strictly more harm than if we over-punish an execute him.

But even in regular case, why should we be humane to someone who refuses to be humane to others? I don't get this logic at all.