I don't believe in the concept of "sin," as in violations of some ultimate, universal, god-given rulebook. I do believe in the existence of evil, human mistakes, meanness, poor judgment, character flaws, violations of human laws (crime), violations of social and cultural norms, and assholes. Atheists, including me, are certainly afflicted with some of those characteristics.

But that's quite different from someone informing me that I am being sinful because I'm doing something that displeases their version of god, and therefore I am a horrible person in a cosmic sense and subject to horrible retributions from said god.