I LOVE Dexter, and here's why:
The main character kills people---yes, that's true. But the way he justifies it almost makes it less horrible. He only kills people that he knows are a threat to society. He sees it as his calling. He satisfies his need to kill and protects the world from criminals. Surprisingly enough, Dexter, in his own twisted way, is a very moral person. He narrates the show, so we see everything through his filter. We see him pretending to be normal, wanting to be, puzzled by the emotional attachments that others have.
But the show is written very intelligently. It never dumbs it down for us. We hear Dexter's thoughts, see what he does--and yet, the scriptwriters have enough faith in the audience to actually let us figure a few things out (gasp!) for ourselves.
There are people in Dexter's life that he really loves, although he thinks he is isn't capable of actually feeling anything. But we can see it. Dexter is smarter, more capable, more talented than any of his co-workers, but he hides it and keeps a low profile. He is always two things at once---a meek public mask he has learned to adopt, and the dark, powerful killer underneath. But he is also just Dexter, a guy who wants to fit in, and wants to understand. In some ways, he really is a good guy. He cares, although he keeps telling himself he doesn't. He feels rage when he sees innocent people suffer, he wants to protect and comfort the people he cares about.
He also has "The Dark Passenger," that drives him to satisfy his need to kill and still meet the strict codes of justice that his foster-father instilled in him. He doesn't kill randomly. He carefully follows trials, researches evidence, examines the blood evidence ( his specialty, of course), and when the guilty go free, he steps in.
Watch the first three episodes. You won't be able to stop.