question about cloony's boss (spoiler)

Pavel (the old guy) wanted Jack killed because Jack decided he had had enough of the detached assassinations he performed. Then Jack kills a witness--Ingrid, a lover, perhaps a friend--at the beginning of the movie. That is enough for him. Jack calls Favel and tells him he, Jack, wants out. From Favel's point of view, Jack cannot leave. If Jack does, then Favel cannot control him. Jack literally knows where the bodies are buried, and Jack could eventually become a witness against Pavel. So Pavel puts out a contract to take out the one guy who can expose himself and his operation.

Trouble is, Pavel is old and his reflexes have slow down just enough for Jack to shoot him in the head despite the fact that Pavel had the drop on Jack.

It wasn’t explained. But you know Clooney did a lot of work for him, they have a history together. You’re led to infer that no one could ever leave the profession, probably because of loose-ends, so that’s why he wanted Clooney dead.