First of all, the question seems to imply that because all Atheists are angry, the Athiest position is somehow invalidated, which just doesn't make any sense. Many blacks in America are angry about racism, but does this mean that the Civil Rights movement is invalid? Of course not. The emotional state of the Athiest is completely irrelevant to the debate, and does not detract or add anything to the argument on either side, so why ask the question at all?
Secondly, the people who ask this question seem to think that all Athiests are angry at religion. Not ALL Athiests are angry. There are enough angry Atheists that one can get the impression that this is how Atheists are. But an Atheist could just as easily get the impression that all Christians reason very poorly and know nothing at all about logic or critical thinking. There are many Athiests who are not angry at all, and never attack religion, even when there might be a good reason to do so, in my opinion. The idea that Athiests as a group are angry is simply an over-generalization.
So, now that my objections to the question have been stated, here are SOME of the many reasons I am personally angry at religion:
I am angry that so many parents and religious leaders terrorize children (who have brains that are hard-wired to trust adults and believe what they're told, and are very literal-minded) with vivid, traumatizing stories of eternal burning and torture to ensure that they'll be too frightened to even question religion, and I am angry that religious leaders explicitly tell children and adults that the very questioning of religion and the existence of hell is a dreadful sin, one that will guarantee them that hell is where they'll go when they die. I am angry at the priests who molest children and tell them it"s God's will, and I am angry at the Catholic Church that consciously, deliberately, repeatedly, for years, acted to protect priests who molested children, and consciously and deliberately acted to keep it a secret, placing the Church's reputation as a higher priority then children not being molested. I am enraged when I think about all the child abuse that children have suffered in "Gods" name because of religion.
I am angry that public policy all over the world on important issues such as same-sex marriage, abortion, stem-cell research and sex education in schools, are being based, not on evidence of which policies do and don't work and what is and isn't true about the world, but on religious texts written hundreds or thousands of years ago, and on the policy makers own personal feelings about how those texts should be interpreted, with no supporting evidence whatsoever. I get angry when religious believers base their entire philosophy of life on what they interpret the Bible to mean, and when they ignore or reject or rationalize any evidence that contradicts the Bible, and then accuse atheists of being close-minded and ignoring the obvious truth, while they try and force everyone one to live their lives based on the same Bible, or any Holy Text. for that matter, instead of letting people live there own lives.
I get angry when believers glorify religious faith without evidence as a positive virtue, a character trait that makes people good and noble. And, FINALLY, I get angry when believers trumpet every good thing that's ever been done in the name of religion as a reason why religion is a force for good... and then, when confronted with the horrible evils done in religion's name, say that those evils weren't done because of religion, they were done because of politics of greed or fear or something else, and, would have been done anyway even without religion, and shouldn't be counted as religion's fault.