The No True Scotsman fallacy and religion: how does it apply?

Today I was reading a debate where someone claimed that Christians have never committed violence in the name of their religion because anyone who does that can’t be a “true Christian”. To me, that seemed like the blatant use of the No True Scotsman fallacy, but maybe I am misunderstanding its definition.

Who is even in a position to define what is a “true” member of a religion? Extremists say those who aren’t extreme like them are not “true” and the more moderate folks say the extremists aren’t “true”.

Is defining someone as a “true member of X religion” just a product of guilt by association? Why can’t we admit that there are bad apples in our group? That would be like me saying the CEO of Abercrombie & Fitch isn’t gay because he’s a piece of shit. But alas, he’s as gay as I am.