Good post, although I don't agree that Denny's as bad as Bobby (though he possibly has the potential to be).
I think the writers have gone out of their way to show Bobby as a bit evil. Remember, when he attacked Lucy it was unprovoked, so I'm not sure it can be explained as a fit of temper. Sure, he was upset that she was causing trouble within the family, but there was nothing going on at the moment he struck her. It seems to have been a pre-meditated act.
Denny has the potential to go either way in my opinion. With good parenting, I think he's still young enough to grow into a decent enough young man (admittedly that's unlikely to happen

).