Coll was actually one of two Irish-American hitmen to go by "Mad Dog," but he was the first by several decades. Coll operated in New York during Prohibition, primarily for mobster Dutch Schultz. Coll was born in Ireland and came to America as a child. He was widely knownfor his absolute ruthlessness, eventually forming his own gang and turning against his former employer.
Coll rose to notoriety when he allegedly killed a young child during a kidnapping attempt - taboo even in the world of gangsters. Coll, however, maintained that he didn't kill the child, and that it was a setup by Dutch Schultz to make him a pariah in the New York crime world. Whether Coll did it or not, "the baby killer" was indeed ostracized, and eventually murdered by a former co-worker in the employ of Schultz. He's estimated to have killed at least three dozen people, but it could have been more.