Evangelical atheism?

I have been on Twitter for a few months now and am finding my way round slowly. One thing is a particular puzzle to me and that is the aggression of some atheists. Their need to proclaim their beliefs and debunk others is very strange to someone as tolerant of other peoples beliefs as I am.
The whole basis of their argument is negative. If you have to tell people you don't believe in God, that fine. But they have made their non belief into a sort of religion in itself.
I am not talking here about Humanists or people for whom any church is a restriction. I am talking about those people whose sole reason for being on Twitter seems to be to attack, to debunk to ridicule. How can you make a religion of hate from not believing something? An evangelical atheist is something I would never have believed possible. But here they are. Why? They must get something out of their non belief but surely the more intelligent ones would be able to see the irony of their position?
A belief in something is easy to defend but a belief that something does not exist is hardly worth stating.