The Amazing Maurice and His Educated Rodents. Random fact of note: This is the first book in the Discworld series that's intended for children. Of course, Pratchett has made it clear that he's all for blood and murder in children's books, seeing as that's what they really like; see the page quote on Disneyfication.
Rat catchers breed strong rats for "the pit", where people bet on how many rats a terrier can kill in a given time, by locking them by the hundreds in cages and keeping the ones that do not get eaten by the others. The threat/risk of Malicia and Keith getting eaten alive by those frenzied rats.
As the leader of the intelligent rats new in town, Hamnpork said (or shouted) "There are rats EATING EACH OTHER in there! Let them out!" And Hamnpork has no problem with eating other rats which have died of, say, traps, so that means that the rats in the cages were either killing each other for the meat — or else not bothering with the killing first.
The rat-catchers do say that the rats were being fed carcasses from the rat pits, actually. Possibly it's the fact that the trapped keekees have no alternative but to eat rat that's so horrifying to the Clan: it's not pragmatic recycling, it's the dreadful algebra of necessity at work.
A less violent bit of creepy comes when some of the Educated Rodents encounter something so horrifying that they forget how to talk- i.e., lose their sapience.
An intelligent rat watching a crowd of humans killing rats for sport in a barn whose doors are currently barred, and which is full of hay. Said rat had just had first-hand experience of aforementioned Pit, followed by a near-death experience in a rat-trap. He strikes a match.
Plus, he'd also been through the Cage Room, which was essentially a rodent's version of a concentration camp.
Spider the Rat King (a group of rats with their tails tied together)
Mind-controls non-thinking giant rats, and sics those rats on Maurice the cat to kill him. At one point Maurice has to get out of a cluttered cellar with his eyes closed, because Spider can see whatever he sees.
Messes with the brains of humans (the rat catchers and Keith and Malicia), and takes partial control of Maurice's body - "The cat will not move. Be still, CAT. Or I shall command your lungs to stop."
Gives Dangerous Beans, the spiritual leader of the intelligent rats, a bloody scary talk amounting to him (Spider) being more or less the God of rats, and planning to annihilate humanity.
"And they live under your floorboards, sir. They know how to use fire. They know all about poison." The graphic descriptions of just what which poisons will do to somebody earlier in the book do not help. Oh, and one of those scenes involved the intelligent rats wanting to put one of their fellows out of his misery, and after some agonising have to put him in a trap.
The existence and origin of the Rat King also begs some Fridge Horror: If every rat catcher is required to make a Rat King to become a master in the guild, and there's at least some chance of the Rat King becoming a mind-reading, mind-raping monstrosity... Just who is actually running the Ratcatchers' Guild by now?
Rat Kings really exist.
As do people cruel and imaginative enough to create them, simply because they have too much time on their hands.