Mafia hitman Richard Kuklinski (The Iceman) tended to kill victims with cyanide, often by accidentally spilling cyanide poisoned drinks on their clothes, while they were at a bar or a club
Richard Kuklinski, aka The Iceman, was a hitman who worked for various crime families, primarily the Gambinos. He has claimed over 200 kills, though that's probably an exaggeration.
Kuklinski was unstable and would often roam around New York City killing people for little to no reason. If somebody rubbed him the wrong way, he would kill them. Early on he killed numerous homeless people to perfect his murderous ways.
Kuklinski killed people for over thirty years. Lack of attention from law enforcement was partly due to Kuklinski's ever-changing methods: he used guns, knives, explosives, tire irons, fire, poison, asphyxiation, and even bare-handed beatings, "just for the exercise."
He favored the use of cyanide, since it killed quickly and was hard to detect in a toxicology test. He would variously administer it by injection, by putting it on a person's food, by aerosol spray, or by simply spilling it on the victim's skin.
Kuklinski was eventually convicted of five murders and served the rest of his life in prison. It took a year-long undercover investigation to finally bring him to justice.