Well, if he's punishing the bad ones, wouldn't that make him good? I mean, no teacher in her right mind puts the worst student in class in charge when she leaves.
If Satan was both 'bad' and 'in charge of Hell' wouldn't he just let people go since they turned against 'God?' Wouldn't he want to encourage that behavior rather than punish it?
So if we can speak 'Christian' for a moment, either Satan is actually good because he's punishing the bad people in Hell, or the Christian God is actually the one in charge of Hell and torture. Nothing else makes logical sense.
Except, of course, that neither of them exist.