Witness Unprotection Program
A criminal hunted by goons hands himself over to the police, but the goons are able to kill him anyway,
Until I can think of a good description, the laconic version says it all.
- The Saint short story "The High Fence". A captured criminal agrees to turn Queen's Evidence and tell the police who the High Fence (an underworld buyer of stolen goods) really is. He's murdered in his cell by being fed poisoned food. When another criminal is taken into custody and agrees to tell Inspector Teal the High Fence's address, he's shot dead before he can do so.
- The Dark Knight: Batman kidnaps a mob accountant and turns him over to Gotham police, and the accountant agrees to help the police bring the mob down in exchange for amnesty. In response, the mob hires The Joker as a 'problem-solver'; Joker proceeds to get himself arrested, blow up the police station with a cellphone bomb, spring the accountant from his cell and execute him by immolating him with gasolene.
- In the Doctor Who serial The Ambassadors of Death, a scientist called Lennox runs away from where he's been held and forced to do alien experiments and turns himself in to UNIT. His superiors are quick to find out where he is and murder him by putting a radioactive isotope in his cell.
- In the Torchwood episode Small Worlds, Mark Goodson, a paedophile, is being stalked by alien fairies (It Makes Sense in Context). He turns himself in to the police, hoping to be safe there, but the fairies get in and kill him.
Hello, Unknown Troper. You'll need to get known to lend a hand here.