The example you gave, based only on what you said, doesn't seem that bad by itself- in fact, it reads more like a moderately shitty case of Cursed with Awesome. I mean, he just gave the dude fuckin' immortality, and the issue here seems to be the reason the vamp did it and how much it violates local norms, and not what led to the guy being turned, the practicalities of what turning him into whatever entailed, or what the dude's life will be like now. The classic vampire myths make mention of turning people into thralls, or mind-controlled not-quite-vampirized slaves drunk on their master's blood, but the fact that his actions are considered merely countercultural and selfish rather than tantamount to rape and enslavement strongly implies that that's not what happened and that your setting's version of undeath isn't that bad, and the scenario doesn't rule out the possibility of the dude escaping servitude or the vampire just kicking him out when he's done and the dude living out the rest of his undead existence in peace. Unless being turned into an enthralled mindless servant to a vampire is actually looked upon in this setting with any positive feelings at all, in which case, you have a source of heinous cruelty already.
So, you either left out some important details, or you need pointers on what makes something truly heinous.
Rather than focusing on fantastical cultural norms that don't really have an equivalent in real life, I recommend zooming in and examining the people involved, and how their lives are affected.
If the vampire actually enthralled the guy, and turned him from an ordinary guy into a braindead slave dependent on his master's blood, most people are going to see that as fucked up and heinous all on its own, and considering that he seems to have done it on a whim, it's probably safe to say that he didn't tell the guy exactly what that would entail or give him adequate time to make an informed decision, so he violated the guy's bodily autonomy, too, and that's going to be an unpleasant parallel for anybody who's been sexually assaulted. For extra shitbag points, he could abuse his thrall(s), knowing they're not capable of even thinking of leaving or fighting back.
As for other shitty things he can do, if he's smart, he'd want to only be a real shitbag to people who can't retaliate against him in any real way, or do things that he can cover up or where he can escape the consequences, because that's how you stay alive. Given that he only took a thrall at that time because he needed one, he sounds like he'd be too busy to actively go looking for shitty things to do, and so his assholishness would mostly come out if you get in his way or if you happen to be nearby and suitable for the next step of his plan. Then again, you haven't said what this guy is actually like as a person. If I were you, I'd look at those other major evil acts of his and determine how and why he did those things. If you find something that ties all of the 'whys' of those acts together and figure out his style from the 'hows', you can extrapolate other, smaller deeds he may have done in the interim, and likely also who he is beyond being an asshole. Assholes aren't assholes all the time, after all.
"Jack, you have debauched my sloth."
Thank you for your detailed reply and advice. You make a good point about how concentrating on the cultural norms of fantasy beings lessens the impact of the villain’s actions, so I will definitely place less of an focus on that.
About how the vampirism affected the victim, I suppose it is Cursed with Awesome to a point. The person in question doesn’t become a mindless slave automatically and does gain a weaker version of the master’s powers...however the master can turn them into the aforementioned witless thrall if they wish and even without losing their mind they are bound to follow their sire’s whims.
As for what the villain was like as a person...well, he was a petty thief and general “thug” as a mortal human and carries this into undeath to a large extent. Essentially someone who preys on and hurts the weak and helpless both for money and food.
Fear is a tyrant and a despot, more terrible than the rack, more potent than the snake. — Edgar Walllace
I have created a character who is, to not put too fine a point on it, a piece of crap. A selfish, evil and cowardly man who contributed greatly to the suffering of the world in general despite not being a "major player" in terms of evil forces in the world.
What I'm stuck on is how to make this character an indisputably bad guy. To explain, his major plot-relevant evil deeds are already set in stone so that makes it clear what kind of person he is. But I'm confused about the little things someone like him would do.
An example of his crimes if someone would like something to go on: The man in question is a vampire. Undeath in this setting is considered a blessing by some and curse by others, but it is a weighty and esteemed thing to bestow upon someone. This villain turns a bystander because the man knew of hiding places in the city and he needed a servant. Zero regard to ancient beliefs and traditions as he holds nothing sacred apart from his own personal pleasure and gain.
Fear is a tyrant and a despot, more terrible than the rack, more potent than the snake. — Edgar Walllace