In Norse Mythology, Loki rarely had it good. He started off as a slightly more happy-go-lucky (for whatever that's worth in Norse myths) trickster god. He ensured that Asgard's wall would be built for free, and aided Thor in many of his journeys. Hell, in some versions, Thor even says that Loki is "an evil man, but a good companion". Sure, he caused a great deal of trouble, but at the same time, he's typically regarded as a permanent outsider (in small part, one might argue, due to his half-giant heritage, never mind that a lot of other gods are half giant), gets threatened, and, at one point, gets his lips sewn shut by some bastard dwarves while all the other gods look on and laugh. Arguably, this would piss a lot of people off. And then, long story short, he murders Baldur, is punished horribly, and then causes Ragnarok, never repenting for his deeds.
This actually gets worse when one realizes that there are TWO versions of the story leading to Ragnarok, and in the other, Loki never killed Baldur at all. He was punished instead for getting drunk at a party and insulting the other guests. For this crime, he was imprisoned and forced to watch as one of his sons is transformed against his will into a wolf which then tore out the entrails of his twin brother, the entrails that were then in turn used to bind Loki into place to suffer eternal torment. That's right: unending agony while bound in the remnants of your murdered son. Yeah. All that for a bit of snark. It's really no wonder that when he gets out, he decides to join the Jotun to end the world.
Of the children he fathered, two are thrown completely out of Asgard by Odin (Hel and Jormungandr, both for being ugly), while one gets chained to an island for the rest of time (Fenris) for being big, strong, and scary... and, essentially, for being a child of Loki (because, you know, you can't trust Loki's kids). The fact that all of these kids come back and help their father destroy Asgard at Gotterdammerung is perhaps understandable. (Fenris is usually told as being imprisoned precisely because the Asgard knew of his important role in Ragnarok, but then it gets into Self-Fulfilling Prophecy territory.)
Some of the monsters from Classical Mythology, especially the ones that used to be humans but were turned into monsters as punishment for some crime. Perhaps the most obvious example is the Gorgon Medusa and her sisters. Medusa got turned into a monster for the "crime" of getting raped. That's right, Poseidon raped her in Athena´s temple. Athena got pissed that sex had occurred in one of her sacred places, but couldn't do anything to Poseidon, so she took out her anger on Medusa by turning her and her sisters into Gorgons.
This is only the Roman version of this story (by Ovid), and most of the others make the situation slightly less bad. In some versions, the sex between Medusa and Poseidon was consensual and in others, like the Greek and dominant version, this is completely averted and the Revenge by Proxy angle is eliminated. The Gorgons always looked like that (which makes sense, their parents being Ceto and Phorcys, after all, and were some of the many terrifying monsters that plagued the unhappy world.
Izanami-no-Mikoto, the goddess of death from Japanese Mythology. She dies horribly by being burned alive when she gives birth to Kagutsuchi, the god of fire; her last act while alive being to give birth to Mizuhame, a water goddess, and instruct her to calm her brother should he ever become too violent. However, her concern turns out to be for naught as her husband, Izanagi, chops the newly born Kagutsuchi into pieces as revenge for causing his wife's death. So, when her husband comes to bring her back to the world of the living, Izanami is understandably pissed off at him and spitefully reveals that she has willingly joined the realm of the dead now. However, she nevertheless promises to ask the gods presiding over the realm if she can return with him, provided Izanagi will not look at her during his visit... Which he, of course, does. Which makes him realize that his formerly hot wife is now a rotting corpse, the sight which makes him flee the realm of the dead in a panic and seal the entrance with a huge boulder. And so Izanami, having had quite enough with all this shit, swears that if Izanagi does not let her out of the realm of the dead, she'll become death itself and kill a thousand living beings every day. Izanagi, remaining as reasonable, mature and accommodating as he's been so far, sneers back that he'll just give life to one-thousand five-hundred every day in return.
Tiamat from the Enűma Eliš. Tiamat and her husband Apsû were the primordial beings of creation; all the gods descended from them. When these gods got raucous, Apsû suggested to her that they be put down. In older translations, Tiamat was all for this, but more recent work has shown that she tried to veto the idea: "let us attend kindly!" So when Apsû conspired with his vizier Mummu, the god Ea overheard, put them to sleep, and killed Apsû. Even then, Tiamat finally only makes war against Ea and his followers when incited to it by other gods.