I watched a few episodes, but I didn't think Dexter was a realistic character. Not that I know how a true serial killer is, but I felt he was overdramatized. That they did everything to somehow provoke the audience, and just make him the most evil person they could think of. I think I would have preferred him a bit more subtle. He could still have the same attitude, but in a more subtle way, so that there were more room for interpreting his behavior. Dexter's reasoning seems like a normal person's, just more "evil". I didn't like his voice either. He might have grown on me if I continued watching, the plot is kinda intriguing. Maybe I'll give it another chance.
Oh, it's been a while since I watched it, so my observations might be a bit incorrect.