I know that serial killers are often methodical and intelligent but the level or restraint that these 2 show is astounding. Although, I'm not sure Dexter is your typical serial killer. I have a half-formed theory that he's like a cross between normal and serial killer...a hybrid of sorts. I am always drawn to Harry saying that he was scared to early, that they thought he could be saved but it seems it was too late (of course that's all a paraphrase but you get the gist. lol).
Anyway, it seems like his serial killer self wasn't fully formed and neither was his psyche allowed to form naturally.