Any atheist who professes to "hate God" generally does not believe in god, hence the term "atheist." If they do, I wouldn't call them an atheist, I'd call them a struggling Agnostic or quasi-Christian or something OTHER than atheist. There are various reasons for people to claim to hate god when in fact they don't believe in the deity at all. Jaez has given some explanation to this, but I'd like to elaborate more.
The first reason to claim "I hate God" is because of god's influence on other people, in other words "I hate the idea of god, and the fact that this idea has controlled people for millenia." The atheist has no true belief in "god" as a being, but god's existence as an influential symbol is very real, and obvious to everybody. Many people are disillusioned with the world, after having come to the conclusion that too many people are obnoxious, stupid, or just plain mean. These bad people seem to create a chain reaction of bad vibes, leading to a perpetual cycle of conflict and hatred that pervades human history. Many atheists believe that the idea of a "god" is what makes humans so disrespectful of vital existence -- mortality, which makes here-and-now life a little less important to humanity as a whole. The atheist may think to himself (yes there are female atheists, I just don't feel like doing the whole hermaphroditic pronoun thing today) "If people didn't waste their time praying to non-existent deities, they would devote more time to developing decent relationships with their fellow man." Examples: gay bashing, slavery (embraced in the Old Testament), racism (not directly praised in the Bible, but that doesn't stop people from using it to somehow justify it), imperialism (many people believe/have believed the Bible gives them an excuse to conquer the world), war (many wars, not all, but many, are religious...the rest are usually nationalistic, which is a kind of godless religion...). Many atheists believe that religion makes man needlessly arrogant while making him feel somehow (usually subconsciously) worthless at the same time. One can only imagine how such paradoxical tendencies can drive our cultural evolution.
Now of course a theist believes the complete opposite. Many theists, particularily Christians, believe that man is inherently biased towards committing bad deeds, and that without god we would have no moral compass with which to guide our futures. Many (not all) theists are convinced that man is a wicked abomination that needs to be controlled from afar. The Bible tells them "you can't even come close to god," "you're a sinner," "you've missed the mark, try again." God is the solution, not the problem, they profess. It is this attitude that man is a "problem" needing solution that gives atheists the reverse opinion. Believing that man is basically a crude and worthless being without a god in the heavens (especially when said god doesn't exist) is only setting the entire human race up for failure. Personally, if I died and lo-and-behold found myself at god's throne, I'd have a hard time not asking "Why did you create me in the first place if I'm 'broken'? And if you created me in your image, what does that say about you?"
As an atheist, I subscribe to this theory, this idea that striving for impossible perfection makes it impossible to be truly happy and cooperative as an imperfect being, whether or not said being believes in god. I personally believe that the belief in eternal life (far separated from this world) has created a planet teeming with vicious assholes, who for whatever reasons devalue just about everything except their own existence. Life is disposable -- everything is disposable. Who cares about posterity? Who cares about the environment? Who cares about the unborn (unless they've been conceived within a womb at this juncture in time)? If we weren't so convinced that there was something "better than this," we wouldn't be so vindictive and so quick to make life out to be a living hell for others and ourselves. It's a conscious attitude that results in a subconscious behavior. The material world is worthless to those who drool at things that exist only in their imagination.
I'm sure there are some people out there who claim to be atheists yet are really frustrated in that they would like to believe there is a god, but they simply can't. They may be Agnostic and may embrace science to such a degree as to say "I can't believe in god because there is no evidence, and everything appears to be an absolutely ungodly mess. But I'd like to think someone is going to fix this." This is a result of a new, aspiring, potential atheist's unwillingness to embrace the fact that the Universe is ambivalent. It doesn't care whether you live or die, it doesn't exist to make your life better or worse. Such atheists suffer from an inner conflict that results from their naive outlook on the world, combined with their newfound lack of faith. Their problem is that they fail to see the beauty of chaos. I would say that this type of atheist is rare, as generally those who want god to exist can make it so, within the confines of their own mind. As an atheist, I find harmony in the fact that while I'm not blessed or cursed by the heavens, I do, remarkably enough, exist, and have been granted an opportunity to do some cool stuff while I do. The fact that I exist makes the chaos beautiful. Chaos admiring chaos is an amazing idea.
Atheists must face the fact that there is nobody out there to dictate how they should live their lives (except maybe their parents). They must come to terms with their own undeniable mortality, and they must develop a morality and ethics based on common sense, one that allows them to stay alive as long as possible by cooperating with fellow human beings (a la tit-for-tat). Most atheists have come to terms with the (disputed) fact that the Universe is not here for our benefit, but that if we play by the right set of arbitrary rules, we just might have a good time while we're here.
As an aside, I think that Ayn Rand's biggest flaw was to believe the Universe is innately benevolent. Maybe that's why she believed that absolute, unrestrained Capitalism would work. Unfortunately the forces of the Universe have no concern for what goes on within our pale blue dot. While I wouldn't want to complicate the definition of "atheist," I think that every true atheist has to come to terms with the idea of Universal ambivalence at one point or another. The majority of atheists I've encountered don't believe in good or evil, except in the cultural context of human civilization. Without gravity, life never would have materialized on Earth. This doesn't mean gravity is benevolent. You can test this easily on your own...