Most atheists I have conversed with have their biggest problem with the God of the OT, spurred on by new atheism and the likes of Dawkins and Hitchens, a new type of aggressive atheism.
How can one be SO angry at someone who "doesn't exist"?
Atheists aren't angry at God. The reason many atheists focus on the OT God isn't because they're mad at him, but because they find his characteristics contradicting, his actions pointless, or his behavior corrupt. Obviously, atheists don't believe God to be real so they're technically critiquing the Bible and the idea of God - not God himself.
In Dawkins case the answer is simple, at aged 13 he was abused by a priest.
There's this common misconception that atheists choose not to believe in God because they're mad at him. This makes as much sense as choosing not to believe in cancer because doctors found a tumor in your body. If a person chooses not to believe in God just because they're mad at him, then they're simply in denial; technically speaking, they're not atheists (but the semantics isn't really important).
It is possible for some people to go into denial about God's existence, where they try to convince themselves God isn't real when deep down they do believe he exists. But this is usually not the case for most atheists. Most atheists genuinely do not believe God is real.
It is possible for anger towards God to lead people towards atheism without them going into a state of denial. Allow me to explain:
For most Christians, God defines who they are. God becomes a part of their identity. This makes it hard for them to reject God because rejecting God would be like rejecting a large part of their own personality. For many Christians, giving up their belief in God would be like cutting off an arm or a leg.
When a person becomes angry at God, they tend to rely less on God to define who they are. When a person doesn't make God a part of their identity, rejecting the idea of God no longer feels like a personal loss. A person who uses God to define who they are might find it impossible to imagine life without worshiping God whereas a person who feels betrayed by God can imagine life without God. And when one can imagine life without God, it becomes easier to accept the possibility that God might not exist.
So do atheists choose not to believe in God because they're angry? Not really. Instead, some Christians who become angry at God find it easier to be convinced he isn't real or that there's no solid evidence for his existence. Anger doesn't lead directly to nonbelief, but rather it leads to being open to the idea that God doesn't exist.
