Based on what we see in this movie, Bobby didn't deserve to die
He was kind of an *beep* but he didn't deserve to die.
Yes, he hit his friend, but it was not like his friend couldn't defend himself, based on what we saw. Marty hit Bobby back afterwards.
As about the rape, I think Bobby didn't realize the girl took it as rape. Think about it: Bobby had a reputation of being into rough sex so Bobby probably thought she was there for that. Of course what he did was wrong and he should have paid more attention, but I don't think he realized how hurt she truly was. It's a bit of a gray area.
Why couldn't they just remove Bobby from their lives? What stopped them? Why didn't the girl just go to the police after the rape? (the others didn't like Bobby so they would have supported her tale about the rape and if they told the police how violent Bobby was, it's more than likely the police would have believed them)
They were stupid when they killed him. Sure Bobby was kind of an *beep* but not irredeemable in my opinion.