I agree, it is foolish to hate a God that you don't believe in. In fact it's a contradiction. There are some other possibilities though, most notably (and rather popular with the Christians and Muslims of this world):
I-Hate-God-Cos-He-Doesn't-Like-Me Syndrome. Which is rather self-explanatory.
The I-Hate-God-Cos-I-Want-Him-To-Exist-But-I-Cant-See-Him-While-Others-Appear-To Syndrome, less common than the first, and often confused with it, but still quite popular among the nascent atheist.
Then there's the I-Hate-God-Because-The-People-Who-Worship-Him-Are-Jerks-And-Ergo-He-Must-Also-Be-A-Jerk.
OR perhaps:
I-Hate-God-Because-The-People-Who-Worship-Him-Are-Jerks-And-I-Like-Irritating-Them
Or perhaps the people who say they really hate society, and confuse the patriarchal feelings that are transferred from God to Society in early teenagerhood, and therefore displace their hatred of society back onto God. It may be worth asking how many of the people who profess to hate God are actually contented happy well adjusted individuals who have good home and family lives and enjoy the company of their fellow human beings.
Just an idea folks.