Athiests always seem to be pretty aggressive, but I guess they're pretty passionate about what other people think.
I don't recall a group of atheists ever starting a war over atheism, or start a terrorist group because they weren't happy with bunch of atheists who might just have a different view of atheism.
On the other hand history is littered with wars fought over religion. Christiantiy tried to convert the world to Christianity. "Hey guys, let's have a little crusade". "hmm, look at all these natives in Africa/South America/wherever, they could do with a good dose of Christianity"."You damn protestants". "you damn Catholics". "Our rules are better than your rules". "our God is better than your God".
Just for the record, if any of these Gods actually existed, Buddha would kick their behinds. Have you seen how much weight that guy is packing, but then again it's the only religion where he's likely to go "you know what guys, you just have your little God war and have a nice day, I'll sit here and meditate while you do". Which probably explains his weight problem, he could have done with a little more fighting and a bit less meditating to shift some of that fat".
Most atheists don't give a rats arse about what other people think and are quite happy for them to believe what they want to believe. Richard Dawkins, as a prime example, has never threatened anyone, never wished anyone dead because of their beliefs. His passion isn't agression. On the other hand he has been threatened by religious nuts opposed to his views, not to mention numerous well wishers expressing the hope that he drops dead.
Religion has no scientific basis, it's much easier for those that believe in it to try and debunk evolutionary theory as they don;t feel they need to provide proof for their own beliefs "it just is". Creationists will point to supposed flaws in evolutionary theory, diseregarding the fact that by its very nature science is about theories being proven or disproven and new theories hypothesised as more evidence is found and knowledge expands.
There is irrefutable proof of evolution. Studies of molecular biology, genetics and cladistics have all yielded a wealth of evidence, though unfortunatley because of the nature of organic matter, the passing of time and the effects of Earth's geology, physics and chemistry, there are gaps in evolutionary history, some of which may never be closed. These gaps are waht creationists try and jump on. The claims of creationists and their criticisms of evolutionary science have been refuted thouroughly by numerous evolutionary biologists and scientists. Books by the likes of Mayr, Futuyama and many others have answered the criticisms, but these are rarely read by the wider public unless you are studying the subject of evolution. So it's easy for creationists to make their arguments with people who may not have the full scientific knowledge and only a rudimentary knowledge of evolution, to refute their claims.
A good and very concise book on evolution is "What Evolution is" by Ernst Mayr. I'd recommend it.
