But, as someone once said (can't remember who), morality is something you do when no one is looking. As such, morals are a set of rules, but between you and your sense of integrity, whatever that amounts to. A system of laws regulates a society and are usually based on a moral code (don't steal, kill, etc.) that the vast majority can adhere to.
Dexter does have his own rules, which, on the surface, appear to be all there is. But as we get to know him better (and as he evolves himself), we see that he does, in fact, have his own moral code. And that was brilliantly brought home by the kid's seeing Dexter as Jesus Christ, his "savior." For me that was both comedic and insightful, which is why I love the writing of this series so much.