There's a lot of anger being thrown around in the religious arena. A lot of people wouldn't consider it an arena, but there is a battle going on. All religions are fighting each other and all religions are fighting Atheists. There are Theists who understand Atheism well enough to leave it alone just as there are Atheists who understand Theism well enough to leave it alone. However, many insults, slurs, and other ethical, intellectual, and on occasion physical attacks are still being thrown about, from all sides. Agnostics can't even stay neutral as they are forced to stand on one side of the line or the other. Anyone standing on the line is ridiculed from both sides and might as well go into hiding. So, as this is an Atheist blog, not Theist, let's focus on the side we're more familiar with.
"Why do Atheists hate Theists so much?"
Actually, this question is relying on a false statement. Atheists don't hate Theists. Most Atheists see religion as a delusion, true, but we also respect of other's beliefs. We'd rather not have those beliefs be the main driving force of those in power, but we don't mind if you worship God, Allah, Thor, or Buddha. If it makes you happier in life to believe that there is a being with more power and more knowledge than you, who has a plan and wants you to be happy with him, then most of us are all for it. We won't try to stop you from pursuing your own personal happiness, wherever you find it.
Unfortunately, these Atheists aren't as vocal as the ones people mostly encounter. The loud, insulting, and sometimes dangerous Atheists are the most pronounced. There are a few like Thunderf00t from YouTube who are benevolent and just want to reveal our truth to any who will listen. Then there are those like The Amazing Atheist, also on YouTube, who is a little more insulting than most but trying the same thing. He, just like Thunderf00t, is just trying to get people to understand what we know and why we see it as the truth. One thing you have to understand, though, is that these two, as well as a lot of the others on YouTube and similar social networks, are not just being informative but must also be entertaining. Without the entertainment factor no one would sit through the boring parts where they actually explain their position.
As for why the truly dangerous and hateful Atheists hate Theists, I really can't say. Why do the truly dangerous and hateful Theists hate Atheists? There are two sides to every coin, but the answer to your questions doesn't always have to be found on the other side. Sometimes, it's a lot closer to home than you might think. Search within your own group, find the ones that hate the opposing group, and ask them yourself. Maybe you'll get your answer.