I can see your point there. However, when you get that impression, I'm not sure you fully understand WHY the atheist is debating. We do not intend to save a soul; we simply see Christianity as something that is illogical and we are trying to show you our view point and tell you why it's illogical. We then look at your response to it - as, perhaps, you could convince us otherwise (most atheists, in my experience, are not steadfastly set at "There is no God"; most are constantly looking at new evidence to see what it says about the existance of a god or not). However, after several attempts at giving a logical argument and receiving something back that doesn't make sense, or doesn't answer the question directly, it can become very difficult not to be angry or frustrated, and it can often show. No offense to Christains, but many of those that I've debated with simply made assertations, and didn't wish to back up their statements.
Oftentimes, even when it does come to someone who is trying to back themselves up, it can be difficult not to get angry at them if they slip up in an argument (for example, if you hired people for a restaurant and many of them were completely incompetent, after a few years of this you might just jump on the new guy that did everything right except he accidentally dropped a plate).
I have yet to read "The God Delusion" - I'm not normally one to read atheist literature because either it brings up examples of extreme stupidity to support it's arguments (and I can't stand reading stupidity, even if it's being quoted and used to show the stupid person is wrong), or it brings up something that I'd like to debate with the author but can't.
Because Christianity happens to be the prevalent religion in the free world, so many atheists are former Christians. I'm not going to really attack Islam that well because I don't know it's arguments; however, I do know what the Bible says, I do know what many arguments for the existence of the Christian god are. It's simply much, much easier to attempt to argue what I already know instead of having to learn a whole new religion and culture to argue it.
I wish I could go more in depth on all this, but I'm not entirely sure if that would break the forum's rules or not.