Searching around, I found an interesting detail. When Jules and Vincent are going to Brett's apartment, they open the trunk of the car in order to get the guns, and this is what they say:
We should have shotguns for this
kind of deal.
How many up there?
Three or four.
Counting our guy?
I'm not sure.
So there could be five guys up there?
We should have fuckin' shotguns.
Vincent says "Counting our guy?", and indeed, they are talking about Marvin. This explains why they don't kill him in the apartment and they take him with them. He's not an hostage, he's an accomplice. This is not a 100% attack-proof evidence, but if he's their guy, it might be that the shot could be indeed an accident.
Furthermore, on the IMDb page about Pulp Fiction, it is stated that:
According to an interview with Phil LaMarr1, it was he who came up with the idea of his character Marvin being shot in the face. Marvin was originally supposed to be accidentally shot in the throat and survive. Knowing that Marvin would die a slow, painful death, Vincent and Jules decide that Marvin should be shot in the head and put out of his misery. Knowing that this would make the characters unlikeable, LaMarr took his idea to Quentin Tarantino and he agreed to it, figuring that a single-bullet kill would be funnier.
This kind of proves the accident way, since originally he was meant to be accidentally shot, not in a fatal way, and then be killed by the two hitmen in order to avoid a slow and painful death. But here the death is sudden, yet still accidental. I'll add the interview if I find it. If someone finds it, please post it.
1: The actor who plays Marvin.