I love Dexter!

I think it's entirely possible. I've read a lot on various serial killers and some of them are/were incredibly clever, highly intelligent people.
Of course, if trained like Dexter, they'd have all the knowledge behind them to cover up their crimes. But, I don't think there are many out there like this though. Perhaps shows like those mentioned highlight this fact and they are more careful, I don't know. But I doubt there are many at all in real life, thankfully! Saying that though, where Dexter is concerned, I'm wondering whether that would be such a bad thing. He only rids the earth of murderers. Sure, that makes him one himself, but he doesn't go after innocent people. Only the bad guys.