"They killed my son." "I'm sorry, commander, but it's true. I don't think Gunder Ironcrust was interested in the politics, you see. He just took a job at the mine because he wanted to feel like a real dwarf and work with a shovel for a few days." "They left him to the mud...any help you need, we will give. Any help. But when you find them, kill them all."
It's only a brief anecdote, and mentioned in passing, but apparently one case that Vimes will never, ever talk about with Sybil involved people who did... something to a little girl. Something that ended with them taking a shoe as a "trophy". The vagueness invites so many horrific contemplations... especially when we're told that Commander Vimes, notorious for his refusal to kill even the worst criminal if he can avoid it because that's murder rather than justice, went berserk and almost slaughtered them upon finally catching them. If Detritus hadn't been there to stop him, he'd have broken his code without thinking to kill them all. So just what did they do?!