The trained killer-for-hire is a fairly standard trope in all kinds of media about courtly intrigue. That way, the king's life can be in danger basically all the time. But did these people actually exist? Wikipedia has a list of assassinations, but all of them seem to have been carried out by secret police or regular people with a grudge like John Wilkes Booth or Gavrilo Princip.
I know about the hashashin, but they seem to be a militant religious order with their own agenda rather than a group of mercenaries that worked for the highest bidder.
I don't care if they were rare, I know that it's not exactly an occupation on the level of "blacksmith" or "hunter." I want to know if they existed at all prior to the modern age.
(I've tagged this middle-ages and renaissance because I wasn't sure what else to add, and these seem to fit the period when these sort of people could have appeared).