These killers are almost always a member of a large criminal organization that deals in illegal goods and services terrorists, mobs, and gangs. Proxy killers convince other people to carry out killings for them.
Ian Doyle, a notorious former member of the IRA, was both a serial killer and a proxy killer. Doyle wasn't afraid to get his hands dirty, but would generally order one of his many men to carry out murders for him.
"Lauren" (Episode 18, Season 6)