Two points.
Killing bad guys is one thing. But dismmeberment and mutilation?
Also, in Dexter's list of bad guys are prostitutes. They don't deserve to be killed. Ergo, he's one of the bad guys.
Prostitutes are not on Dexter's list of bad guys, at least not in the book, and the first episode followed the book almost verbatim, so I assume he won't be after them in the TV series, either.