Violence and Treachery from the sinful asks meme?
VIOLENCE - A time my muse attacked someone unprovoked or without reason.
For all the bad things he has done in the past, he actually hasn't attacked anyone entirely unprovoked, verbal or physical. On the rare occasion he actually goes off on someone, there is always a reason, and for the most part, it takes a lot. Damian is a fairly laid back individual, usually letting things roll off his shoulders, but even so, it is not impossible to drive him to the point of actually attacking somebody. It might take a lot, but it can happen. And just like everyone else, some days that is easier to achieve than others. He's got pissy moods as much as anybody, and yes, those are the days when he very well might attack somebody, but it would still take something to spark that fire.
TREACHERY - A time my muse betrayed someone after building up a relationship.
Now, this is more Damian's style.
While he does have some relationships that are legitimate - ones he actually cares about, makes efforts in/for, and continues to build as time ticks on - there are a number that were built for the sole purpose of betrayal. If he sees something in a person that might be used as blackmail later, if he gains information he needs and has no further use for the one in question, if he already knows a dirty little secret and plays the part of a caring friend and ally only to gather as much Intel as possible before he turns that "friend" over and reaps the benefits - it's all a game to him, deciding who is worth his time and who is worth only whatever he might gain. And in a hundred-plus years, he has learned to play fairly well.
As for a specific incident, there was one a few years back, when faced with a coterie and everything that has to bring with it. Along with him were four others - two Toreador, a Ventrue, and a Malkavian, and all were about the same age, same level of experience of the nightly goings-on within Kindred society, and all that jazz...with the exception of the Ventrue, Ethan. He was marginally younger than the rest, yet in the short span of his life as Kindred, he had achieved immense amounts of success, knew things he probably shouldn't (especially considering he no longer had a sire guiding him ), dabbled in questionable things and presented vast knowledge of those things, and showed a great deal of influence that...well, that part wasn't suspicious necessarily, but it did play into things later on and with further thought.
Naturally, Damian was curious what the deal was.
No one with half a brain in their skull is going to admit to their secrets to just anybody, so he played his hand carefully. As the pair worked together, they grew closer and closer, and eventually the Ventrue legitimately considered Damian a trusted ally and friend. Eventually, he spilled the beans that he had some association with another who had been guiding his hand, offering advice and resources and promises of power, and all it took was cooperation. At least, at first. It came to light that this associate, or advisor as Ethan referred to him, wasn't exactly...well, let’s just say he dabbled in a bit of infernalism. Which is, of course, a big no-no. Ethan kept it quiet, telling only a couple of his closest and most trusted companions - Damian included. Damian kept his mouth shut about it for a short period of time, solely to gather as much information about this person as Ethan was willing to share. Where exactly the guy was located was of specific interest, and once he had that, Damian and his Nosferatu friend (one of those actual friends for whom he’d burn a city down, if the need ever arose) took care of two problems: they went after the infernalist and anyone else sharing his nest, and also staked Ethan out on a rooftop to wait for the sunrise.
This was an extreme and well-deserved example, and the latter is not always true when Damian opts to “switch sides”. Hell, he has turned on people he’s built relationships with just to prove a point in the past, but the above was a more recent instance.