Lamashtu, full stop. Besides being a goddess of monsters and nightmares, there's all the horrifying details surrounding her progeny and worshipers. For example, Lamashtan priestesses who give birth to children blessed by their goddess do so by letting their offspring tear their way out of the womb.
More fun in this vein are The Motherless, Tieflings with Qlippoth heritage. They're implied to eat their way out in childbirth, with invariably fatal results.
Also Zon-Kuthon, god of pain, who preaches torture, mutilation (of both oneself and others), and dismemberment of living victims (who are kept alive as long as possible). Zon-Kuthon brutally tortured and flayed his own father until he was a broken and twisted slave.
His champion as well, the great blue wyrm named Kazavon. He took over the Hold of Belkzen disguised as a human, then he went Vlad the Impaler on everyone. Besides that mess, there's the fact that, in a way, he's still alive.Bringing together the Relics of Kazavon will resurrect the dragon and... well, that's just one more way that Golarion is screwed.
Most of Rise of the Runelords, with a generous side order of Squick. Special mention goes to The Hook Mountain Massacre (part 3), which turned the comically bumbling D&D ogres (traditionally just sword-fodder for guys who'd mastered killing orcs but weren't ready to kill giants) into beings truly worth of the Always Chaotic Evil label.
Carrion Crown is a swift trip into the horror genre. What does it start with? A haunted prison, what else!
Scared of Hillbilly Horrors? Pathfinder takes the tropes and applies them to not one, but three kinds of giants.
First up, there's the Ogres, from their first appearance in "The Hook Mountain Massacre". They're hideously deformed, rampantly incestuous, sadistic, murderous, cannibalistic monsters whose "society" revolves around essentially nothing more than food, sex and torture. Their idea of games include mig-a-mug-tug (grab each other by sensitive spots and yank as hard as you can; first one to collapse in pain loses) and man-swords (smash two humanoids together until they've been crushed to a pulp). Their genes are so polluted that not only are "ogrekin" invariably distorted and grotesque-looking, the ogre's genes effectively destroy the bloodline — an ogrekin can mate with humans, but nothing in their family tree will ever resemble a human again. Worse still, the rampant inbreeding ogres practice can eventually lead to them devolving into "degenerate ogres", creatures so foul and hideous that even other ogres think of them as primitive monstrosities. To say nothing of the mutations that can plague given clans, like the Shaggras, whose whole bodies are covered in carpets of thick, greasy, rank Prehensile Hair.
Then there's the Hill Giants, who are less incestuous, but still rampant cannibals, brutes and barbarians.
The Marsh Giants, meanwhile, are believed to have been hill giants... once. Now they're something so foul even ogres regard them with fear. They basically practice all of the same horrors as ogres, but with their own horrors on top of it. For example, not only do females spend so much time chewing on toxic mushrooms to enjoy the drugged out states that ensue they invariably either neglect their children or retard them with their poisoned milk, fathers and mothers typically eat their children, since their barbaric form of animism preaches that offspring are parasites of the soul. They also worship Dagon, adding some Lovecraftian cultist action to the mix.