"No True Scotsman" is a funny thing.
Atheists don't (or shouldn't) believe there is any real difference between a Christian and a non-Christian. It's a sliding scale between atheism, agnosticism, casual belief, and fanatical belief. Nor is there any reason to believe that one camp is really better in every possible way.
Lots of fanatical Christians *do* believe there are two camps - the "real" Christians, and everyone else. That's why they play the "No True Scotsman" game - most of them actually believe there is a real difference between "real" Christians, and the rest; and that "real" Christians are morally better in every way. So if a "real" Christian does something (or worse - believes something) they don't agree with, they obviously weren't a "real" Christian.
(OK, that's a bit of a simplification - they still think that "real" Christians do bad things, but then God forgives them and they try not to do it again ... but they should certainly all *try* to live by the same moral code, since God is guiding them).