...Strahd's entire backstory is literally the girl he was stalking preferred his brother over him. Soth wouldn't trust his wife. D'Honaire was a spoiled brat with innate hypnotism powers. Aderre and Boritsi both had mommy issues. Dilisnya, Inza, and the Three Hags were all born evil. Von Kharkov wanted a girlfriend but ended up killing them because he's a cat.
They each had committed terrible deeds that they refused to acknowledge, let alone even attempt to make up for... but the reasoning behind it was rarely that "adult-level."
Heh, well, boiled down it does sound a bit like people I went to school with (except the killing part) like JQ said, lol.
But what pushed Strahd over the edge was (1) his belief the natural process of aging was the reason Tatyana preferred his brother and saw him like a father, (2) his anger and rage that he could not overcome old age, (3) his anger and rage that someone who never saw blood and battlefield should deserve to "win" a prize like Tatyana, and (4) an arrogance that he "deserved" that prize because of his sacrifice and service.
This simmered nicely in a pot for years until he was willing to sell his soul to roofie a woman into being with him, murder his own brother, and not give a damn about anything but his own need to win and prove his entire existence wasn't merely a shadow of the real thing. This is the delicious environment that the Dark Powers then put him in: a land of soul-less carbon copy shadows, mocking his shadow existence by living alongside him in his personal hell that, even to this day, he firmly believes he can overcome...this time.
But that's all too much for a product that has to sell as PG and fits better into a novel.
Going back to the OP's original question that I dodged earlier, I classically liked Lamordia.
It's Dr. Frankenstein, but what I like is that too often PCs may see bad guys as bags of hit points to be solved. You're a boss, so I kill you, loot the body, and get the XP. Sometimes there's just evil locked away in its own private little hell, and there's nothing to be solved. Adam and his creator are eternally bound in their love-hate relationship. Like Strahd, the Dr. is incapable of understanding he's just as responsible for killing Elise as Adam, that his life is a failure, and that Elise is gone (replaced by a shadow that simulates just enough life to make the Dr. continue to harvest "for her sake").
There's a horror in knowing that no matter what you do, you're not going to solve this. No matter how much sense you make, the Dr. won't see differently. He can't. No matter if you destroy everything, the Land and Dark Powers aren't letting go of their playthings, and they'll just come back. You can't "win," at least not in the traditional sense you're used to in other settings where you kill the bad guy and the good guys prevail. Sometimes, like in Barovia, the victory comes in creating even the tiniest bit of genuine "hope," the one thing a Land of horror and despair seeks to quash.