Why do most people think atheists are angry and hateful?

We don't suffer fools gladly. And if you want to see anger just try to point out the inconsistencies in the Old and New Testaments or refer them to evidence that both books had many authors over 1,500 years, that they reflect the cultural and political opinions of the times of the various writers and that not only is there evidence that any prophecies were fulfilled there is evidence that these prophecies were written after the fact. You want to see rage? Try getting them to actually do research on their own religion.