Cheer up, ZS. Serial killers need love, too.
Dexter may be a serial killer, but he's easy to fall for. He tries so hard to be a good father and shows his vulnerabilities, yet he's strong and committed. The show wouldn't be outstanding if we weren't rooting for him to kill the bad guy.
I think it's both, but definitely more the character we're attracted to, once we go beyond the physical, as their current character is how we get to know them.