The sin-eater is a veritable karma houdini -making machine.
There's also the story about the knight who has to find out what women most desire. He has a year and a day to find out, or he will be executed, and eventually promises to do the next thing an old crone asks of him in return for the secret (they want power in their relationships). She demands he marry him, and it turns out that she can make herself young and beautiful again. He lets her choose whether she will be beautiful in the day or night(or, depending on the version, beautiful and unfaithful, or old and true), and in return gets a beautiful wife all the time. Why did he have to find out what women most desire? He raped a girl, and in return gets a beautiful wife for it. There was absolutely no other punishment besides "find out what we want most and you can live."
It may not be the origin, but that story was included as the Wife of Bath's tale in The Canterbury Tales. For extra ridiculousness, the one who offered the knight the most insane pardon for rape ever was the Queen. The King just wanted him killed.
It wasn't quite that simple, he would be killed in the end if he couldn't figure out what women want. After searching and asking many women, who all gave him different answers, is when he found the crone. I believe she tricked him into marrying her and gave him the choice when he was presented to the queen again, where he gave her his answer, he says "whatever you decide, wife" basically giving the woman her own choice. Because he recognizes the will of the crone she and the queen forgive him, so the crone will be a crone by day, but at night turn into a beautiful woman for him and also stay true.
The Jerkass king in the Rumpelstiltskin story who threatens to behead the miller's daughter unless she spins straw into gold. He gets the gold and the girl.
The Yellow Dwarf and the Fairy of the Desert in Madame d'Aulnoy's Fairy TaleThe Yellow Dwarf get no comeuppance after they have led Princess Toutebelle and the King of the Gold Mines to their deaths.
Several other fairy antagonists in Madame d'Aulnoy's stories get away with either placing curses on princesses/princes or holding them captive. The Fairy of the Spring in The Hind of the Wood goes unpunished for cursing Desiree to not see sunlight, and Ragotte in The Ram triumphs once the prince she turned into a ram dies.
Many fairies who put curses on princesses/princes go unpunished. The evil fairy in Sleeping Beauty goes unpunished in Perrault's version and in the Brothers Grimm's version (in contrast to Disney's version).
In Greek Mythology, the goddess Nemesis was all about preventing Karma Houdinis. In fact, she was the goddess of karma.
Though the Greek Gods themselves are apparently exempt from Nemesis' Karma. As long as they don't offend a god that's more powerful then them, (which happens only occasionally) any of the Greek Gods can do whatever they want to both each other and mortals. Neither other Gods or especially mortals are able to do anything about it. In fact, if a mortal is involved in the slight, even if it's not their fault, (i.e. Medusa or the various women Zeus has cheated on Hera with), it's always the MORTAL who gets punished.
How about the story about when king Midas was supposed to judge if Apollo or Pan was the better musician, and Apollo punished him with having donkey ears just because he liked Pan's music better? Yikes!
There are a few times when Jerk Ass gods go too far and upset Zeus himself, which pretty much always ends badly for them thanks to Zeus's status as Greek Mythology's undisputed king of Disproportionate Retribution. Zeus himself, of course, is never punished by anyone.
Medea. She first fell in love with Jason and helps him fulfilling the task her father gave to him. She used her magic to make her love reciprocal (Jason was already married to Hypsipyle). After she decided to fly with him, in order to delay her father, she killed her brother Absyrtus cut his body into pieces and strewed them on the road. When she found out her husband was about to marry another girl, she killed the latter and all the children she herself had with Jason. Later she also tried to poisoned Theseus, the son of Aegeus she remarried and managed to escape once again. There is no known story of her being punished for all her crimes. for some authors, she died in Colchis and after her death ended in the Elysian Fields (the paradise) where she married Achilles.
Some versions portrait Medea as a less evil person, where she only killed her brother through an accident, and Jason was a jerkass, who wanted to leave her for another woman. It goes at least back to Euripides, who made her a sympathetic character in his play about her. He even let her talk against the oppression of women in ancient Greek society!
Medea pales in comparison to King Minos. The guy steals youth from Athens, who had done nothing to harm him, and had them fed to The Minotaur, but when he dies, he becomes a judge of the dead, not only does he escpae punishment, he's REWARDED.
While Loki did eventually get punished for his crimes, it wasn't until he outright bragged to the faces of the other gods that it happened. Before that he got away with literal murder and the Norse didn't do more than tut tut at him.