I think you can use the same type of thinking to say Stalin was no secular humanist. I don't think you can get him out of the atheist camp very easily. Atheism is too clean a concept to give you much wiggle room. There's not much tied to it. It's just a single position on an single question. Well, maybe you could find some agnostic-sounding quote and move him to the agnostic camp...I don't know. Christianity is loaded with baggage and opposing Christians that don't think the other Christians count as Christians and people that argue that your actions either do or don't reflect what truth you believe in and all that jazz.
edit: I get you're not seriously trying to argue Stalin wasn't atheist. What I'm really getting at is that the no true Scotsman thing specifically goes after circular arguments. Sometimes there are things that disqualify you from a group so it is possible to misuse the argument...I really wouldn't hold Stalin against someone pushing secular humanism.
What I'm getting at is that just because someone goes against what their religion/non-religion teaches, that doesn't disqualify them from that. If that's the case, does a Christian who lies, steals, cheats, has pre-marital sex get kicked out of the club, too? If sin determines whether you are a Christian or not, then nobody is a christian. If only certain sins cause exclusion, then you need to find a passage that explains which ones, because that's not in the book I have read.
That pastor can ask forgiveness, change his ways and get back into God's good grace any time. Same goes for any Christian who sins. This pastor is a Christian. He's just an asshole who commits the worst of sins. Just as Stalin was an atheist, but the murderous, genocidal kind. Just because of his actions, you can't deny his beliefs.