You are among politically-minded individuals who range in flavors of leftism. Then one of the members says that they used to be a neo-Nazi. How would you react?

Driving people out because of their past isn't fruitful. Someone used to have bigoted views but something made them change their mind, and they pushed hard enough to reform their life? Sounds like they wanted it, and are probably a changed person.

I know a few people like this. They help others leave similar situations. Great people doing important work.