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Big Tiny (Theodus Crane), Tomas (Nick Gomez) and Andrew (Markice Moore)
Death episode: "Sick" / "Killer Within"
How it happened: After Rick's group successfully took the prison in the Season 3 premiere, they learned they weren't the only people living there. Tensions with the untrustworthy prisoners were always on high, but Tomas brutally killed Big Tiny after the big guy was bitten by a walker. Then Rick brutally killed Tomas after he nearly got Rick killed during a walker battle. Rick chased down Andrew and thought he left him to die in the courtyard, but it turned out Andrew survived and unleashed the walker onslaught that claimed Lori and T-Dog's lives. He was ultimately killed by a gunshot from fellow prisoner Oscar.
Shock factor: Rick slicing into Tomas' head with a machete was the most shocking death, but Andrew's nefarious payback was the most shocking event caused by this trio of bad news guys.