Others have already mentioned Jesse James and John Wesley Hardin – both notorious, cold-blooded killers.
I’ll mention a guy who’s post-Old West.
Rafael “Red” Lopez (1886–1921). Lopez was thought to have committed over 30 murders.
Lopez was the son of a soldier in Pancho Villa’s army. He grew up as a miner in New Mexico and Utah and became a skilled horseman. He was hired by Buffalo Bill for his traveling show in 1913. Later that year, though, Lopez murdered another Mexican. When a posse tried to arrest him, Lopez shot and killed three of them. He killed two more lawmen during another attempt to apprehend him.
Lopez escaped Utah and moved to Texas where he formed his own outlaw gang and also fought alongside Pancho Villa. In 1914, Lopez’s gang ambushed a train and murdered 19 passengers.
After the train massacre, the Texas Rangers, led by Frank Hamer pursued Lopez but he avoided capture for years. It was 1921 when Hamer got a tip that Lopez was having a meeting somewhere. The informant told Hamer where Lopez would be and Hamer set up an ambush. However, after looking over the area, Hamer got suspicious, thinking it was a trap. He ordered his men to move to a spot 30 yards away from their original position.
Sure enough, Lopez and his gang, along with the informant, approached the site where Hamer had been. Hamer yelled at them to surrender. Lopez fired and grazed Hamer but Hamer fired back, shooting Lopez through the heart and killing him almost instantly. 11 of Lopez’s gang members were also killed. It turned out that Hamer shot right through a gold watch that Lopez kept in his breast pocket.