On one hand, if Marvin had an opinion, Vincent likely wouldn't have had reason to turn around, and thus would not have shot Marvin in the face. On the other hand, Marvin worked for Marcellus, but he still seemed like a loose end. I don't think Jules and Vincent would have just let him walk. It probably would have been cleaner, but I think Marvin's time was done as soon as he forgot about the he guy in the bathroom with the hand canon. It occurred to me tonight that maybe it wasn't even an accident. Maybe Vincent didn't want him talking to anyone.

Or maybe Tarantino was just trying to make the point that you should always take a stand, and don't not have an opinion.