There’s better answers here addressing the unlikeliness of contract killers existing in the sense of a freelancer available for hire to all the general public rather than someone trusted by peers in a criminal enterprise and also about the “contract” not really being a contract because it’s unenforceable but I think you could also draw the conclusion just by reasoning alone that if a contract killer in that Hollywood sense of the term existed, and breached their contract and you the client don’t have the personal connections or power to threaten that particular assassin’s life in response, there’s always just good old reputation at stake. I mean, it wouldn’t help you in your specific case if they’ve nicked off with the money and left your target very much alive but to have hired them in the first place would have required word of mouth in hopefully a pretty small community of people since you can’t exactly expect to find them on fiverr so you could make it know pretty quickly that this person doesn’t honour their contracts so their wet work career would be over pretty quick.