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I don’t think it’s selfish. At all.
If you let him know you’ll be there for him whenever he decides to leave the cult, you’re pretty much being generous. You don’t owe him forgiveness, but offering it up front might also make it easier for him to one day get out. And it might make it easier on you as well.
“Yeah, sure my friend here is a nazi sympathiser, but he respects me”
It’s important to be tolerant and to respect other people even if we disagree on things. But there are limits to that.