I never really believed in anything, but there was a time when i really tried because that's what i thought was right at the time. But admitting to others that i was confused about the whole thing made me less and less accepting of religion. I was told i was going to hell, that my future children were going to hell, that i would never be loved, that i deserved to be tortured and to die a horrible death, and that i obviously didn't qualify as a person because i had no soul.
Being only 13 at the time, it really got to me and i decided that i wanted no part of it. All the comments i had received in hate soon fed my hate of those who had said such horrible things and for a few years, i despised religious individuals and was so angry that they believed i was less than human because of my confusion. If a god did exist, then i wanted to have no part of him and determined that there was no god, at least not for me.
Now, many years later and i am still wary of religious people but have no more hate. I opened my heart to people with other opinions because i did not want to become what they had been for me. Now i have many religious friends and am constantly learning about new things.
But, sometimes it is easy to be so angry when you have all the memories of hate directed toward you.
When I was a teenager, I thought much the way you do. I was a Catholic then, and as I explored religion, I became a non-denominational Christian, then an agnostic deist, and now I'm an agnostic atheist. I subtly moved from my original faith to my current one on two basis: a moral one, and a scientific one.
From a scientific standpoint, I embraced a non-religious outlook on the world because it seemed the most rational, and it still does. The world can be succinctly described from what we know scientifically, through observation, testing, and the formation of theories. The only truth that religion has had on reality, on a scientific basis, are either things that have been dis-proven (no religion in the past has ever predicted The Big Bang Theory or the Theory of Evolution) or things that may or may not be true (such as the existence of supernatural elements).
From a moral standpoint, I found (surprisingly enough), that despite what many apologetics will tell you, evolution does, in fact, have a basis in morality, in the sense that moral behavior (kindness, compassion, charity, etc.) can help a species survive and prosper, whereas evil behavior (acts of theft, murder, and selfishness) will drive a species into the ground. I read and reread my religion's scriptures, and I found that there really wasn't much there to claim morality upon (the Bible, for instance, never asserts that pedophilia is wrong). I found that a simple cross between a person's inherent conscience and their ability to reason morality makes for a better basis of right and wrong than any book.
So, now to be a little more direct with your question: why do atheists seem "angry"? The answer is is that most us aren't. We live happy lives like the rest of the world, our disbelief in gods doesn't change that. What does make an atheist angry periodically, however, is when society asserts religion on it's people when it has no right to do so. The scientific community, the one that has improved the length and quality of our lives, falls under attack by religious populations constantly, being accused of lies and exaggerations whenever the scientific outlook of the world questions the religious outlook, especially in regards to the Theory of Evolution, The Big Bang Theory, and the Plate Tectonics Theory. All three of those theories are not just "mere guesses" like some would assert, but unifying collections of indisputable facts in the fields of biology, astronomy, and geology (respectively). There are those that assert that without belief in God, no one has any basis for morality, as if the human conscience and human rationality isn't enough. It is asserted that we need the bribery of eternal rewards for our good deeds, or the threat of eternal punishment for our evil ones, to stop us from becoming murderers, thieves, and sexual deviants. And this might sound like an exaggeration, but: [link]
So, I wouldn't say that the atheist is an angry person. I'd only say that an atheist has different things to be angry about than most people.
(And if you're referring to atheists on the internet, need I remind you of the existence of trolls, who get a bit of fun from agitating people? )
As mentioned previously, it's the audacious tendency of certain religious people to push their beliefs onto anyone within view. Many look down upon those who don't believe as they do and I think it's ridiculous. Ironically, if they're members of a monotheistic religion, they disbelieve in every other religion that isn't theirs. A lot of the time, to simplify things to them, I just mention how atheists just believe in one less god than they do, and that their feelings towards the other gods they think aren't real are exactly the feelings atheists have towards every religion.
Atheism is merely the disbelief in any deity/supernatural being. There's no set list of views that all atheists share and I think that's a fact that a lot of people are ignorant of. The most misinformed religious people think atheists are "immoral" "angry" or "Devil worshipers". Which goes to show just how little time people spend actually learning about the things/people they supposedly "hate".
As an atheist, I personally find it illogical to believe that there's an invisible man in the sky who created everything, watches everything we do, hears everything we think and is all powerful. With all the knowledge we've acquired through science on subjects such as abiogenesis and evolution, I think many religious people just like the comforting feeling the thought of their "God" brings , and that's fine if they have those beliefs personally. My issues lie when those people persist in bigotry, belittlement and violence towards others who think differently (especially when those who disagree with their views provide evidence and respectful explanations as to why they think the way they do - but they decide to call them names rather than consider what's being said).
I think some of the perceived "anger" is warranted. If you've taken the time to actually learn about concepts possibly explaining how the world works, ask questions and process information subjectively - then be bombarded with messages and people that persist in varied delusions with no evidence to back them - and have the nerve to attack YOU for trying to think logically, you'd have every right to be angry. Having to keep your views quiet out of fear of how your peers and family might react, having to force a smile and falsely agree to religiously bias "advice" people try to give you, being subject to all sorts of scrutiny and misconceptions by misinformed people are all pretty good reasons to be "angry".
When you hear about laws where you can go to jail for a year for not believing in God you tend to get pissed off at religious people (this happened in Kentucky). Other than that I don't get angry at religious people often unless they're pressing beliefs on others, providing false information about science and spreading preconceived notions about atheism/other beliefs. Generally when you're on one side you tend not to look in depth at the other side and figure out why they believe what they do and this starts conflict. I don't mind religious peoples beliefs though.
However, the Angry Atheists you speak of are what we call "New Atheists." The kind of Atheists who would blindly bash religion just for the sake of bashing religion and citing people like "Richard Dawkins" and "Sam Harris."