But he is also a Loveable Rogue, and although he breaks the law and costumes all the time, he is charming enough for the audience to root for him (remember kids, if you are a pretty white boy you can do no wrong!).
He often proposes that Murder is the Best Solution to most of the problems they face. Many times this creates more problems than it solved, because he frequently overlooks stuff like whether he's actually capable of killing the foe, whether they are actually the source of the problem or whether they have friends who'll come looking for revenge.
He might have sworn to give Stefan a lifetime of misery, but he will allow nobody else to mess with his kid brother!.
Because he is a vampire, and as he says, a predatory species, he does not feel bound to human morality, and displays a morality of his own. (AKA justification for being an evil psychopath, despite it being clearly shown that not all vampires are evil.)
Invoked with he wants his beloved to be happy, because he isn't willing to sacrifice his happiness for Elena's - he doesn't want her happy unless it's with him.
Then there are his Forgotten Powers: in season one he could control a crow and fog - he even boasts about it to Stefan. Never been mentioned since he killed the crow. Not to mention the selective superhearing that works on some episodes, but not others....