What's wrong with religious people?

How come some people (atheists) try to show people how they think their religion is ridiculous and why they should not believe what they do? And then I don’t mean the annoying kind of religious people that do the exact opposite of this (i.e. trying to talk everybody into thinking their religion is king), but just people who might mention in a conversation that they’re religious. I mean, it’s not harmful, is it?

I bet you thought this question would be of a whole different nature before you read the description ;-)