In order to settle a large debt, an exiled hit man agrees to a dangerous but lucrative assassination in Vegas, where nothing goes to plan after he realizes he’s been conned by his estranged brother-in-law.

Thanks everyone

"With a large debt on his head, an(something)assassin agrees to lucrative hit in Vegas. Things fall apart when he discovers that the mark is his estranged brother"

Probably not your story, but you get the idea. Best of luck!

Correction: That's "What about the possibility" rather than "Why about the possibility". (Doh!)