Why be mad at fictional characters? That seems a little odd to me considering how angry some atheists can get. As a Christian I'm not mad at Odin, Athena, or Vishnu despite not believing in them. Most adherents of other religions strike me as misguided people who mean well and are perhaps future Christians. In every people group you will find criminals and extremists, including among believers and non-believers, so I don't automatically assume that's what they are.
Perhaps some atheists are mad at the universe, as the universe is similar to the concept of a monotheistic God, and they're looking for a scapegoat. Or more charitably maybe they don't like others defining the creator of the universe for them. The latter seems more sensible to me. I consider one's relationship with our Creator to be deeply personal, and having freedom of thought is important to me. Besides, some of them end up finding God in the process. Seek, and ye shall find.
Then again, others are actively rebelling. As with a rebellious teenager it's more a matter of emotion than reason. It seems to me that being mad at fictional characters requires a great deal of misplaced emotion. That's why I don't think there are many reasonable people among the most famous atheists. They tend to rely on being emotionally charged rather than demonstrating basic disbelief. Some believers do this too, so it's not like they're inherently more emotional than others, but it hurts the argument that their lack of belief comes from a place of objectivity.
At the end of the day though, atheists are individuals like the rest of us and life is a journey.