Be very careful with that line of thinking. It amounts to 'he's a bad person, so we should punish him by assigning him low status, and part of that is to refuse to take his opinion seriously.' It's instinctively appealing, but it's also an area where human instinct is terribly wrong.
A better way to react would be 'it looks like he's done bad things, so when he talks about those bad things, he knows what he's talking about and we should take his opinion very seriously. Whether he should be punished is a separate question, but if so, that punishment should take a form that doesn't cripple our ability to stop the other people who are continuing to do those bad things.'