Marv motive? (Huge Spoilers)
I got the impression that Marv intentionally let himself be killed. Like suicide by cop, only his was suicide by mob assasin. I got the impression he didn't care about living anymore - didn't care about seeing Europe, bills piling up, Chechan mob having zero respect for him. Maybe he had a life insurance policy his sister could cash in on.
Marv was so calm (and unarmed) and didn't make a move as the hitman walked up and shot him. He was so reckless with his plans and actions toward the end, and you know he was a street-smart guy to have survived the mob that long. Just seemed to give up. I think he engineered his own assasination.