The rude obnoxious atheists that post here (NOTE: MOST ATHEISTS ARE NOT LIKE THIS..none I know in real life are) were indoctrinated with religion as children, perhaps oppressed by the dogma, made to feel like sinners for expressing quite normal behaviours.
So their frustrated rants in the religious section are about projection. They want to project the feelings of inferiority that were instilled in them as children and probably still carry into adult hood onto those they feel most to blame...the religious. Of course it's really their parents they blame but their indoctrination won't allow them to kick out at them. In fact their parents probably are unaware of their atheist views.
Then there are those that as fathers they enjoy telling their children Santa doesn't exist, they get a morbid pleasure out bursting the bubble.
The atheists that post here but are NOT rude and obnoxious fall into many camps, those that are against oppression and they see religion as being oppressive and dangerous.
Then there are those that 'believe' in science to such an extent that nothing exists outside of it until science says so.
There are also so many false ideas tied into many religions that some feel compelled to dispute them. No disrespect intended.
Most people I know are atheist, they are all loving, kind, tolerant, generous, compassionate human beings and regardless of their views re theism and religion would quietly keep them to themselves out of respect, but this would be different if we lived in a culture where religion was oppressive and affected our quality of life. Where I live it does not.
I also know Christians in my environment, and they are very community orientated and do lots of charitable works, and are non judgemental. So it really is more about people than 'labels' and people make up all groups.
There is good and bad in every group.