From Wikipedia;
In his life Thompson killed five men. The first was in 1910, in rural Arkansas, when a man called Jim Johnson accused him of cheating at dice and threw him off the boat on which they were traveling (and which Thompson had recently won when gambling with its previous owner—a friend of Johnson's). When Thompson climbed back on board, Johnson drew a knife and threatened Thompson's girlfriend, who was also on board. Thompson seized a hammer and struck Johnson several times on the head before throwing him overboard. The unconscious Johnson drowned. Thompson showed no remorse, stating that it was Johnson's fault for not being able to swim. The sheriff gave Thompson the choice of standing trial or handing over the deed to the boat and leaving town, which he chose.[6]
The other four men Thompson killed were shot by him in self-defense when they tried to rob him of gambling winnings. Two were killed in one incident in St. Louis in 1919 (the local police chief thanked him for killing two wanted bank robbers).[6] The third came in St. Joseph, Missouri where Thompson and his hired bodyguard between them shot two men attempting to rob a poker game (again, the victims were known criminals and no charges were pressed).[6] Thompson's last killing came near a country club in Texas in 1932 when he shot a masked figure who was holding him at gunpoint. This turned out to be sixteen-year-old Jimmy Frederick, who had caddied for Thompson earlier that day in a winning match. The dying Frederick confirmed to witnesses that he had been trying to rob Thompson.[