Dexter - yes, to graphic scenes of killing. Also graphic scenes of death and body parts, sometimes, depending on what season you're in. To date, no icky animal abuse (in fact, in one episode, Dexter and Rita advocate to save a neglected dog). Everything that would fit into the R category, but not go above into NC-17 territory. So they are either brief scenes, or only shown once per episode, etc.

I would hate to think I've missed something, but any animal cruelty is referred to and not shown. Dexter as a child refers to hurting the dog because it wouldn't shut up, but you never see anything (not even a flashback scene), and by the time he's an adult, there are no animal cruelty scenes.

He's also not a serial killer in the classic sense that he's not a classic sociopath (though he's conditioned to believe he is). So while he was cruel to animals as a child, you actually find that he is quite able and capable of defending animals, children (his first victim is a pedophile) and sometimes even humans as an adult.

Hope this helps!