Atheists are a small minority in the United States, where society tends to give religious irrationality a pass. That alone is enough to make a person critical, judgmental, and sarcastic. It's not necessarily productive in changing society; it's a way for atheists to feel like at least they're not standing idly by.

Self-directed negativeness is not, in my experience, a trend among atheists. It may be a consequence of the atheists you know not having found their own coherent philosophies after rejecting religious ones, or maybe they haven't realized there's nothing wrong with having something to protect. The step from atheism to humanism (or rationality in general), from ceasing to believe in the afterlife to realizing just how valuable our lives really are, is a non-trivial one for most people.