Sasuke amuses me greatly.
But seriously, I think he's believable enough, but it's hard to say because we don't see a lot of his thought processes. Not everyone in the world is rational or mentally well-balanced, and not everyone ends up making the "right" decision in the end. Most people are not the self-sacrificing, I-care-about-others-more-than-myself heroes that the Narutoverse is full of. It's true that in Naruto, everyone and their mother has a tragic backstory, but honestly, how many people in their situations would really come out of it all unscathed? At the very least you're talking a lot of depression, anxiety and warped worldviews.