Why didn't Vincent and Jules kill Marvin?

It's never explicitly said in the film, but if we listen closely the apartment scene we can understand Marvin was their informant within that group. Here is the script of the scene.

Jules - "We should have shotguns for this kinda deal."

Vincent - "How many are up there?"

Jules - "Three or four."

Vincent - "That's counting our guy?"

Jules - "Not sure."

Vincent - "So that means there could be up to five guys up there?"

Jules - "It's possible."

When Vincent says "That's counting our guy" Marvin was their guy who's the informant. After shooting Brett and the others, Vincent says to Jules, "Do you know this guy?" and Jules responds, "Yeah". This is why they didn't shoot him.

They did not kill Marvin because he was part of Wallace's network and was an informer in the gang.

Remember they asked "why didn't you tell us there was a man in the kitchen with a god damn cannon?" or something like that. This shows that they were expecting him to provide this info and logically he was their man (mentioned in the hallway).

If they had killed him then there would be little incentive for people to inform on gangs that had betrayed Wallace in the future. This is also, probably, why Jules and Vincent are uninterested in the excuses "You're ass ain't talking your way out of this" because regardless of what they say the hitmen know their true plan from Marvin.

According to Phil LaMarr (actor who played Marvin), from his AMA (Ask Me Anything) post on Reddit /r/IAMA, it's because (emphasis mine):

The back story that Quentin and I talked about was that Marvin and Jules had people in common and when this briefcase thing went down Jules was able to get in touch w/ Marvin and offered these guys who were holed up, scared and obviously way out of their depth a way to resolve this. "All you have to do is make sure the door is unlocked at 7:45." He didn't know exactly how it was going to be resolved and he wasn't street-smart enough to figure that the resolution might have a significant cost.

I think that marvin was his informant, but was going to try to double cross Jules and Vincent. THAT'S why the guy was in the bathroom with a gun. Jules himself says, we should be dead. Divine intervention stopping the bullets? That's why Vince gets so upset at Marvin for 'forgetting' that the guy was in the bathroom with a hand cannon. Jules' attention is focused on that they weren't killed. Later, divine justice is laid upon Marvin for his attempt at a double cross, when he gets shot in the face.

I think that, if Jules attention hasn't been distracted, they might have killed Marvin in the apartment. Because of the total screwup.

Marvin was a nobody. Signified by Wolf later, who described him as 'nobody who'll be missed'.

Just my opinion.

Marvin was an informant, when you first see Vincent meet Marcellus earlier in the movie at the bar , as they hug Vincent whistlers,”man I’m really sorry”. And Marcellus replies “you shouldn’t worry about it” as he looks over Vincent’s dorky clothes. why would vincent apologise? They got the briefcase back which was their mission. My assumption is that they were supposed to bring Marvin back as he was a known associate of Marcellus but Vincent accidentally shot him, that’s why he apologises.

Well for starters, Marvin was their informant and Brett, Roger (guy on the couch) and Man #4 (hand cannon guy, guy in bathroom) were the targets.

Man #4 could have survived if he had stayed in the bathroom. Anyhow, Marvin was not purposely killed because he was ratting his fellow pencil-neck college students out so he could live.

A choice he later regretted (as you can see when Brett is killed) but due to Vincent Vega's poor gun safety skills, Marvin gets his head blown off.

Also the reason it all happened in the first place was because Brett and his friends refused to give up the brief case for whatever reason.

I think Vincent killed Marvin on purpose. If you look closely at the movie scene. When Vincent had the gun pointed at Marvin you can see his figure squeezing the trigger slowly.

I’m convinced that Vincent accidentally kills Marvin. This is evidenced by several mistakes that Vincent makes throughout the film; each mistake a product of ego and stupidity. He is simply careless. The first thing that comes to mind is Vincent at Butches apartment. He leaves a loaded gun (with the silencer attached noless) in plain sight on the kitchen counter while he is in the restroom, resulting in Butch shooting and killing him.

Another incident that comes to mind is when he is with Mia and he leaves his jacket unattended (with Mia, a known cocaine abuser) with an 8 ball of heroin in the pocket for her to find. This takes place after he becomes frantically worried (before the date) about making a mistake on the date that could piss off his boss (Mia’s husband).

In addition, in an effort to save Mias life after she ODs on the heroin, Vincent drives like a complete idiot over to his dope dealers house to reverse the OD. His driving could have easily attracted the police and led them to a dope house. Finally, the fact that Vincent accidently shoots Marvin in the head IN THE MORNING, ON A BUSY STREET, and in a moving vehicle WITHOUT TINTS only perpetuates his inadvertent stupidity. If Vincent wanted to kill Marvin on purpose, I would think that he’d wait to get to a more familiar area or, at the very least make less of a mess by “accidently” shooting him in the chest.

Anyways, it’s just a movie and it’s just my opinion. Thanks for reading.