In all the cases you mentioned, these assassins were not in a business or contracting service, they were government agents, military, and otherwise trained and in the sole use of their own government. In most cases of “mafia” hitmen, they were not contractors either, but essentially a specialist within a family. There are freelance hitmen, especially in places like Mexico where cartels hire from pools of desperate young men, who get paid mediocre money for killing. I doubt that a world of truly skilled, high-end assassins exists as a free market, too many legitimate and illegal interests would benefit from their dissolution.