I have met some angry atheists though, who are angry at the very people who taught them to believe. I have never been angry at this. The people who taught us to believe didn't know any better. They just taught us what they were taught to believe. They did not think they were teaching us to believe lies.
So what am I angry about? Several things off the top of my head (in no particular order):
I am angry at ignorance and uncritical thinking.
I am angry at what the church has done in the name of God.
I am angry at the oppression of women due to religious beliefs.
I am angry at the suppression of scientific progress.
I am angry at the political power of the religious right in America.
I am angry at the witch-hunts taking place in Africa at the hands of evangelicals.
I am angry at how some believers demonize non-believers today.
I am also angry, in some sense, that I basically wasted much of my whole life on Christianity, as a student, preacher, defender and now even as a debunker. I hope my story and my arguments can keep many young people from going down the same path so they can enjoy a more fruitful and productive life (not that I haven't experienced this to a great degree).
In some ways I picture the very people who suffered greatly at the hands of a once powerful church so that I can have the freedom to speak out against these kinds of injustices. I will not let their blood be shed for nothing. I picture the sufferings of the heretics during the Inquisition, or the medieval witch hunts, and it motivates me to speak out on their behalf.
Again, I'm not angry with God or believers as a whole. I'm mad as hell at the delusional and barbaric beliefs they hold to which cause brainwashed believers to commit horrible deeds in the name of faith. If you were to see it from my perspective you might be mad as hell too.
Khan in the Star Trek movie The Wrath of Khan expresses this anger very well. Okay, okay, maybe that's a little over the top. ;-)