For many atheists, nothing. We aren't angry at what seems to be a fictional character from a book. We may get irritated at annoying religious followers, though.

The atheists who were always atheistic, this can't even apply.

For those who "deconverted", a bad experience may be a catalyst, however, mostly they stop believing as they evaluate the evidence (or lack thereof), over a long duration of time.

Many theists like to believe that atheists are just having a juvenile, emotional, knee-jerk reaction to a bad experience, and don't really disbelieve - just rebelling.

For many atheists, this simply is not the case. We arrive at our position through logic and reason, more often than not. Some become atheists for emotional reasons, however, they typically "bounce back" to theism after awhile.

Often, those who are skeptical in nature just tend to question the doctrines and dogma. When it does not add up, they begin to doubt it more and more.

As a theist, ask yourself why you're mad at Zeus or Thor. The answer is that you're not. Other, more rational reasons exist for not believing in a supernatural entity.