Because when they see good, they expect good. And I don't want to have to live up to anybody's expectations.

Damon in a nutshell. He spends most of his life as a vampire shutting out his humanity and he puts even more effort into making sure that people know he's not human or a good guy or basically anything other than a cold-blooded monster who doesn't care about anyone or anything. Unfortunately, Damon's a shitty actor and a shitty liar; Elena's the first to see through his façade, but she's not the only one. It says a lot that by late S1 and beyond, people (Alaric, Isobel, Rose) who meet him only briefly recognize just how much he cares for Elena and his brother. It says even more that he spent 145 years devoted to rescuing Katherine. Damon is a creature consumed by love, no matter how much he might deny it. He was that way as a human; as a vampire, heightened emotions and senses dial everything up to eleven. He can be both at his best and his most destructive when he does something for love, but it's definitely not something he does in moderation. Damon in love is like Stefan on human blood—at his core, it's what drives him.

This is not to say, however, that his façade is only a front. He genuinely believes he's not a good person or capable of doing good things. It's a role he finds a twisted sense of comfort in because it's easier, familiar—there's no one to depend on him, no one disappoint, no one he'll fail. He can't when they don't expect anything from him. Accusing Damon of having humanity is like poking a bear with a stick. He might not do anything, but then he might break you in half just to prove you wrong. Even as late as mid-S3, Damon tells Stefan he's "better at being the bad guy," after deliberately making sure he's the one responsible for killing Bonnie's mother, effectively absolving Stefan of that and, in his mind, allowing Stefan and Elena to get back together. It's not as selfless as it sounds—Damon takes the fall because he'd rather ensure Elena hates him than to have her as a friend who might love him, but deep down he believes she never will. It's a screwed up kind of logic, but there it is and it's classic Damon behavior. He fucks things up, whether because he thinks it's inevitable or he's just throwing a tantrum, and then he goes around not being sorry except he obviously is and it obviously upsets him that he's lost a friend, he just refuses to say it (until he finally does). Damon is, essentially, a hormonal teenager if hormonal teenagers also had homicidal tendencies.

Around the canon point he's coming from, Damon's sort of…mellowed out. Sort of. If it can be called that. Basically, he's just a bit less quick to lash out by killing everything in his way. It doesn't mean he isn't willing to do a lot of terrible things to save Elena's life, but there's nevertheless some progress when, unlike the first time Elena rejects him, no one gets their neck broke (2.01) and Damon does not, in fact, lash out violently at all (3.19). He's trying, in other words. It's a slow road and he's not incapable of falling back into his old ways, but he's not the same person he was a year ago, either. He's spent most of the year being forced into a position where people are relying on him, especially Elena, after Stefan took off to eat people and look for werewolves with his BFF Klaus, leaving him as one of the few who could protect her. Besides, when hybrids and original vampires are trying to kill you left and right, there just isn't time to snack on strangers anymore. He's got better things to do and more important people to kill.

There are three significant people in Damon's life. The first is Elena. She's the last person who should've extended her hand to him—he's nearly killed her and her friends on multiple occasions—but she did and it means something to him. Damon doesn't exactly have a lot of people who would consider him worth saving. Along the way, he fell in love with her, but he also knows she loves Stefan. It results in a push and pull where he wants her, but he also continually pushes her away because it's easier when he knows she'll never go for him. There's nothing worse than hope. By the end, her final rejection is enough to give him a sort of death wish where he nearly lets Alaric kill him until a memory of his first meeting with Elena gets him back on his feet. More important than his love for her, though, is Elena’s friendship, and his love is pretty wrapped up in that as a whole. He couldn’t feel for her the way he does if she wasn’t the friend she is to him. He trusts her and he knows she trusts him now, too, and they’re both bound by how much they care about Stefan, by how much his going off the rails has affected them both. She’s the first person he ever trusted, in fact, and with the exception of Alaric, she’s the only one. Damon doesn’t trust easily (he definitely doesn't trust anyone who's betrayed him before, which Elena has) and he’s never had someone he could genuinely call a friend before her. He likes her. He makes her laugh, he enjoys her company, he recognizes that she can make really reckless decisions sometimes and he won't hesitate to call her out on that. She isn't just some girl who looks like Katherine to him; she's someone he cares for as a real person in her own right.

The second is Stefan, Damon's little brother. They used to be extremely close until Stefan’s blood high finally split them apart. Damon resents his brother greatly and there's nothing false about how much he enjoys messing with him, but as per his deathbed confession, he’s come to acknowledge that he’s blamed Stefan for events that weren’t necessarily Stefan’s fault. Even prior to that, he was willing to risk his life to save his brother, taking on a house full of vampires much older than him just to rescue his baby bro. He loves Stefan. It’s transparent as hell, though getting Damon to admit it is like wringing blood from a stone. This does not, however, make Damon any less exasperated with Stefan in general, whether Stefan is a self-loathing squirrel killer or a humanity-less dick, and he's happy to stab Stefan to make a point. They're slowly finding their way back to each other, though. Damon's openly acknowledged that Stefan is all he has and he's put in a lot of effort in trying to get Stefan back on track, helping him control his bloodlust. Stefan's family. There's no doubt that Stefan is the most important person in his life whether he loves him or hates him or both.

Finally, there is Alaric, Damon's best and only friend. Initially in town to kill him, they developed an uneasy alliance that grew into a real friendship over time. Damon's killed him twice, turned his wife, and is a shameless dick, but somehow they like each other. Even Alaric can't explain why the hell he's friends with Damon. Alaric is a demonstration that Damon can care about someone without having to be desperately in love with them. He spends about three episodes trying to get back into Alaric's good graces after he snapped his neck in a fit (whoops?), and he's the only person aside from Elena to whom Damon's openly apologized to.

These are all deeper issues, though. On general level, he is cynical and sarcastic with a tongue that cuts deep, though he can be charming when he wants to be. Plus, he's easy on the eyes and that hardly hurts his case. His mood can be unpredictable with a temper that manifests fast and cold so that by the time you've realized you've pissed him off, you're already dead. The only exceptions are when he's genuinely hurt—after Katherine left him, for example—in which case he just loses it and breaks things. He's also manipulative to the nth degree. It's telling that he manages to gain the trust of the very people who are specifically looking for a vampire. Damon is reckless, but he's not an idiot (for the most part.) He's a schemer and a tenacious one at that: if there's something he wants done or someone he wants dead, he will keep at it no matter how impossible or how many setbacks until he succeeds or he dies trying.

In short, Damon is the definition of morally ambiguous. Although he’s getting better about his disregard for collateral damage, when it comes down to it, he will let people die if he has to. "There's a small list," he says in regards to who he cares lives or dies. He won't hesitate to kill for and he won't hesitate to die for the few people he loves. Once he's decided someone's worth his loyalty, he's there to the end. Anyone outside that list? It's a coin toss whether he'll bother to rescue your ass because he happens to be feeling something that moment or kill you himself because you've just been unlucky enough to catch him in a bad mood.

» vampire teeth (fangs + blood floods the whites of their eyes, also darkening the veins beneath the eyes)
» vampire blood (if ingested, capable of transforming someone into a vampire if a human dies with the blood in their system; also heals human injuries) > capable of healing only minor cuts and bruises in-game.
» enhanced speed/strength/reflexes. To the human eye, their movements register as a blur at best. > reduced so that he is only a bit quicker/stronger than the average human.
» heightened senses (sight (night vision)/smell/hearing) > he can see a little bit better at night (I'm thinking shadows mostly?) and his hearing is more sensitive than average so he might catch small movements if it's quiet enough, but he's not going to be listening in on whispered conversations from across the room. Ditto for his sense of smell.

Requesting that his sense of smell for blood remain just as strong, however, if that's okay? It won't really be an advantage since people and zombies are bleeding and dying all over the place.
» compulsion (form of mind control, requires eye contact and spoken orders. can be fought, but only in very extreme circumstances.) > will no longer work properly. I'll leave it up to players whether they want it to work or not and to what extent if/when he does use it, and Damon probably will avoid using it since canonically when you're not drinking human blood it can go wrong easily and he knows that.
» dream manipulation (can control others' dreams) > see above.
» accelerated healing (can recover from cuts, stabs, bullets and broken bones anywhere between seconds to minutes.) > minor cuts and bruises heal immediately; larger injuries will take a bit faster, but something like a broken bone will still take weeks (but not months like it normally would). Deeper wounds will take anywhere from days to over a week.
» immortality > nerfed to fit the game resurrection rules.
» protection from the sun due to a magic daylight ring > keeping this one, of course.

» fire or sunlight if his ring is removed/de-spelled
» vervain (poisonous to vampires; also provides immunity to compulsion. not that important since i doubt anyone will show up with this stuff, but it's here just in case. Damon has built up a slight tolerance to it.)
» wood (extremely painful and will kill if stabbed through the heart)
» werewolf bites (lethal)
» invitations (the owner of a home must invite him in or he cannot enter) > in-game, if anyone sets up what could count as a long-term home (i.e. they're planning on staying there for at least a few months) in any abandoned house/building, he will need to be invited in.