People do turn atheism in to a cause for which they could potentially be willing to kill or die for. People can have the same kind of devotion to atheism that they have to any religion. They can believe that "the end of faith" is a goal worth pursuing. They can conclude that this end (of faith) justifies the means. People sometimes assume that heaven and hell make fanaticism possible, and since they are missing on atheism, it isn't possible to be a fanatical atheist. In Communism there was an equivalent to heaven, it was the socialist paradise promised by Marx, and people did kill and die for that. Hell for some atheists seems to be the obstruction of scientific progress, or religious taboos against certain types of sex behavior.
It is also possible for atheists to disregard solid evidence because of their anti-religious convictions. I see little difference between Todd Akin's refusal to accept the existence of pregnancies produced by rape and Richard Dawkins' claim that a religious upbringing does more harm that child sexual abuse. In both cases, the evidence against these claims is overwhelming, but ideology trumps inconvenient facts in both cases.
I do think that the Columbine case underscores another point, that the abandonment of traditional religious belief will not necessarily produce cheerful humanists. I remember when I was growing up there was a 18-year-old boy who killed 5 women with a knife at a beauty parlor, and what people could remember of him from high school was that he was an atheist. Now, there's nothing inevitable about this, but there is a route from atheism to nihilism to murder, and there is also the possibility of crimes of fanaticism to serve the end of faith and the advancement and glorification of science. Of course, there is also the religious justification for the 9/11 attacks, and for the crusades and wars of religion, but these are also not inevitable.