Because atheism is more about what a person denies than what they affirm, it is actually very wide in its scope. Atheists can preach and practice a great range of different things, while remaining just as much an atheist as any other. In fact, there is only really one way to say "this person is not a true atheist": That person believes in some form of divine being. If you don't believe in some form of divine being, you are an atheist. Therefore, it is ridiculous for any atheist to dismiss any other atheist as a true atheist.
Yet, somehow, they often do. Many atheists have a tendency to dismiss atheists who attribute something negative to their atheism. Atheism, for example, necessarily leads to Nihilism, and many atheists admit that. However, many atheists are not Nihilists, and will dismiss atheistic Nihilists as atheists.
Ironically, they even do this with regard to the atrocities argument. Christianity is a codified religion. That means we have Scriptures telling us everything we need to know about faith and the manner of life. We can tell you what comports with our religion and what doesn't, because for the past 2,000 years, we've had a book that tells us. Thus, if you find a Christian who thinks black people are lesser beings, or that we should fight a holy war against Muslims, or that you should kick your children out of the house if they come out as gay etc. we can say they are not Christians, or at the very least are not consistent with their faith. We can not only tell you this, but show you exactly where the Bible says "He who does not love does not know God, for God is love." (1 John 4:8). But an atheist cannot do this when it comes to atheists who are racist, or who support wars to end religion, or who kick their kids out of their houses for things they find offensive. Yet, somehow, they try.
The irony is, it doesn't even matter. The argument was never "this atheist is evil, therefore atheism is evil". It's "this atheist is an example of someone who lives consistently with atheism." Atheism is amoral, not immoral. Atheism doesn't command, but permit. If you're an atheist, you are quite entitled, by your own worldview, to live any way you want. Maybe, like Penn Jillette always says, you "rape all you want", and all you want is not at all. But maybe you want a little bit more. Or maybe a lot more. In atheism, there is no difference. Both desires are equally valid, and whichever you choose has no moral implications. What you do is just what happens.
And as for the meaning of life, that is a figment of our imaginations. You might be valuable to yourself, you might be valuable to your loved ones, but you are not valuable. As I like to say, half the things on your plate valued their lives too. And if you happen to be a vegetarian (because atheism does allow that), things still died, or got ousted from their homes, just so you could eat. To really drive this point home, consider how important Christianity is to people like me. There are three options here:
1. Christianity is true, and thus it is objectively valuable, and evil to attack.
2. Atheism is true, and thus my religion is as valuable as your life. That is to say, not at all.
3. Some other religion is true, and Christianity is as valuable as that religion says it is; probably not very.
Obviously, the first option is the case. We won't give a second thought to option 3, since that would take too long. But if option 2 is the case, you have to acknowledge that subjective value is really no value at all. If it was, then to attack Christianity is hypocrisy (which, of course, atheism allows...). If subjective value matters, then criticising Christianity is still evil, as is not criticising Christianity, since Christianity criticises other worldviews that are valuable to other people.
You see, then, how atheism self destructs, and not because of the people who claim it. It self destructs because, as a worldview, it is completely unliveable. You can dismiss evil atheists for what they do, but the fact is, they are atheists living in accordance with their atheism.
Tragically, the Christian worldview is also unliveable, but not because it self destructs. Christianity is unliveable, at least in this life, because we have all inherited Adam's sinful nature. Just as Adam rebelled against God, so also do we rebel against God. We may preach His laws, but practicing what we preach is almost impossible. We are not perfect. And that is the whole point of Christianity.
See, though we are not perfect, and deserve punishment, God loves us. Thus, He sent His Son to live as a man, and in becoming a man He lived a perfect life. After that, He went to the cross, and on that cross, He bore the sins of the entire world. Mine, yours, John the Baptist, even Adolf Hitler, Jesus bore those sins. The result is that everyone who confesses Jesus as Lord, and believes He rose from the dead, will be forgiven, and receive eternal life, as if they had never sinned. Those who fail to do so, sadly, have their debt returned to them, receiving eternal justice.