Spudfuzz, that isn't necessarily no-true-scottsman. Some suggest that the definition of Christianity excludes such people, whereas others believe it does not. If it does, than it's not no-true-scottsman. If it doesn't, than it is such a fallacy. The thing is, the definition is a matter of opinion, so you will have a lot of people arguing over it.
First off, your icon is so adorable <3 But yes, I absolutely hate that, it's rather hard to define some religions and such when there are many kinds of groups within it, and others will say "group A isn't part of the whole religion because they believe this" while said group will say they do because they believe beliefs X, Y, and Z are the most important beliefs of the whole religion. I stopped really calling myself a Christian because I didn't believe in the Bible, and felt the religion would be more accurate if the Word NEVER changed during all of its history, and well... But yeah, I've gone over to Deism. It seems ridiculous that of a very large belief system, people feel they have the ability to define who is and isn't part of the religion or belief system. I still wonder how extremists don't find their actions extreme, even when going against much more firmly established tenants of the belief (what ever happened to "thou shalt not murder?").
Christianity is a system with a code of conduct. It is supposed to be about love and courage and a belief in God as someone who is patient with the shortcomings of his creatures. To be a Christian means that you live (or make an effort at least) to live by that code of conduct. Thus, not living by said code = not a true [so and so]. It's a valid argument.
Atheism is not an organization. Communist governments commited the mass murders to further their agendas. The parties involved claimed atheism despite several of them being originally supported by their practiced religions and attempted to remove whatever influence relugion had over the people.