Atheism is just another belief system and the idea that a person could be militant about it seems ridiculous.
I an understand a person being agnostic. That is believable. But atheism is about absolutes just like other faiths.
Anyway, it is a known amongst Atheists that some are militant and then why would the be militant or maybe a more descriptive term is a zealot.
Here is an example of a famous case. There are others....
I think that his motivation is more to get even with his Christian wife and less about being an atheist. I have seen interviews with him and he was “driven” and “angry about something.”
http://www.sfgate.com/news/article/Atheist-dad-ready-for-date-at-top-court-2804688.php