atheists are bitter because they know that that according to their own belief system, their lives have no purpose and no meaning and after they expire from their pointless lives, all that awaits them is nothingness.

On top of that, many of them are also bitter because although they never admit it out loud, they realize deep down that the Bible is correct and that they will one day have to give an account to God for their evil ways, and so they are terrified of what will happen to them when they come before the Judgement Seat of Christ.

So all of that bitterness manifests itself into the constant mocking/insulting/hateful attacks against God, morality, and anyone who does not share their worldview. When they do that, atheists are patting themselves on the back and giving each other winks & nudges as if to say: "Although our existences are pointless, and although many of us are secretly terrified at having to one day give an account of our lives to God, at least we can make ourselves feel better by calling ourselves more intelligent than those who do not share our beliefs."