"And also by being marioguy so he hasn't gained any respect from us. So why should we give him an extra chance?"
Because he's human. We all have our flaws, and while I won't defend his earlier actions, we all make mistakes.
You're right, as Marioguy, he hadn't gained any respect. Yet, he's willing to try now. If we were to get rid of him now, we won't ever have the chance to give him our respect.
Try to put yourself in his situation for a moment. I would definitly hope someone would be offering me another chance.