Aren't atheists just atheists because they're mad at or hate God?
Believe it or not, I've heard this one too. "Atheists are just angry at/hate God." In a word, no. An atheist, by definition, doesn't believe that there is a deity of any sort out there. What this question describes is more a fallen theist, one who believes in a given deity but resents or loathes it and thus refuses to follow it. By contrast, becoming an atheist is not an act of rebellion or defiance. It is a matter of coming to the conclusion that there is no god. In other words, the one description is a person who is convinced that there is a god and yet refuses to follow it for some reason; the other is one who concludes that there is no god.
So, atheists are not atheists because they hate God (capital 'g', the "person"), in the same way that most people acknowledge that faeries, unicorns and Zeus are fictional -- not because they hate them (okay, True Blood faeries are an exception), but because...they're not real.
None of which precludes that some atheists will additionally hate the god concept (probably for any number of reasons, including the blanket immunity "men of gods" are granted by their followers, the horrible impact some believers have on the world in pursuit of other-worldly paradises, etc.) or even particular gods. But atheists who hate the god concept or a given god do not believe there are any real gods; they simply believe that the fictional category of gods, or a particular fictional being/its teachings, as nothing more than concepts humanity has devised for its own purposes, are generally bad for our species, lead to rampant abuses, or are otherwise disagreeable. If, for instance, I had some reason to do so, I could detest the thought of goblins or cave trolls all while recognizing that they are fictional. You might watch True Blood and thus come to detest faeries -- all without ever believing that they genuinely exist. Or you might come to detest a set of movie/book characters (I'm not naming names, but they sparkle and sulk...) based on the rabidness of its fanbase -- but not believe vampires and werewolves actually reside in a small town in Washington. It certainly is possible to have intense feelings of dislike for the impact a fictional thing might have on those who believe it or support it, without believing that the fictional thing is real. Now, while this is in the range of possibilities for an atheist, it certainly is not necessary to be an atheist or part of the bargain. It is, as said before, something an atheist might do additionally, but certainly not something that can be inferred by virtue of an individual being an atheist.
With the above stated...I should note there might be individuals who call themselves atheists yet profess to believe that capital-'g' God, the deity, exists. At risk of sounding as if I am invoking the "True Scotsman" fallacy here (the definition is unambiguous, and in no way subjective; thus, this is a binary evaluation, not a dogmatic one), it must be stated that it is, by its very nature, a contradiction in terms for an atheist to believe in the existence of any gods, just as it would be for a theist to believe that there is no deity. An atheist is :
: one who believes that there is no deityIf an individual actually does believe that a deity is real, he or she is not an atheist. Why? Because that is all there is to being an atheist. There is no other dogma attached, no other beliefs that necessarily have to follow. Obviously, individual atheists have their own world views, morals, etc. -- but that is outside of atheism. Atheism is an (hopefully informed) opinion on the existence of deities. Nothing more, and nothing less. If you do not hold the same opinion on the existence of deities, you are not, and cannot, by definition, be an atheist.
To conclude, atheism is not an expression of approval or disapproval of a deity or the concept of deities, but a disavowal of the possibility that they might exist. As with all things, some atheists will approve of the concept in general, or particular myths, some will not, and some will be neutral. That is, however, particular to the atheist and cannot be inferred merely from the fact that he or she is an atheist. It is furthermore ignorant to presume to know the reasons a person has for coming to the conclusion of atheism, and arrogant to attribute base motives to the individual. If you're genuinely curious, ask. But do not presume that just because someone disagrees with you, he or she is a bad person running from the "truth" (which, really, is what the question implies). That is neither an informed nor tenable position.