Warning: This article contains SPOILERS for Marvel's Spider-Man.
Marvel's Spider-Man PS4 ending is something that gamers may not expect, but it works exceptionally well because it circumvents traditional superhero video game tropes and employs a more cinematic conclusion. One month before the game's main story begins, Peter Parker takes down Kingpin for what appears to be the last time, after at least eight years of the two iconic characters duking it out with each other. Because Wilson Fisk is not only responsible for the organized crime that plagues the city but also keeping the rest of the criminal underworld at bay, his arrest created a power vacuum that engulfs the city. It's worth noting that all the villains in the game weren't gunning for Fisk's position, though. Instead, they were led by people who wanted to see Norman Osborn be taken down.
Most of the game follows Spidey's quest to bring Martin Li, aka Mister Negative, to justice, by investigating his crimes and thwarting his plans along with seeking evolution as Doctor Octopus - the game's real villain (more on this later). Throughout Marvel's Spider-Man game, Spidey tries to defeat Mister Negative, who is out to see Norman Osborn lose everything he loves.
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While it's easy to ascertain that Martin Li leads the Demons in their pursuit of defeating Osborn, seeing the Demon mask that Peter had obtained seemingly became the last straw that sent him over the edge, to ultimately accomplish his goal by bringing down the man that not only turned him into who he was but the person who was responsible for killing his parents as well. From then on, Peter's journey unfolds in two ways: one, as Peter Parker, and the other as Spider-Man. Otherwise, the story wouldn't work without both characters in use. Although there are plenty of missions to go through, the story starts to come to a head after the attack at the mayor's re-election rally, which sadly kills NYPD Officer Jefferson Davis.
Why Martin Li/Mister Negative Wants To Take Down Norman Osborn
In the story, it's revealed that Martin Li was an early test subject for the chemical formula/substance that eventually became known as Devil's Breath. He was experimented on in 1986 and the failed test inadvertently gave him his powers, of which one is the ability to create Inner Demons simply by touching them. Sadly, the lab accident resulted in both of Li's parents being killed, for which Li blames Osborn. Although he tried moving on from the event, the pain caught up to him - and he took it upon himself to finally destroy Norman Osborn, once and for all - but not by simply killing him or removing him from power; Li wanted to take everything away from Osborn, and he was going to use Devil's Breath to do it.
It's not explicitly explained, but it's presumed that this failed experiment is what resulted in Octavius taking a settlement from Osborn and leaving Oscorp for good. But it's later revealed that he's been helping Li keep his powers (and inner demon) under control by treating him on the side. That relationship later comes into play when Octavius extorts Mister Negative's powers for his own gain: to defeat Norman Osborn. Unlike most comic book villains, Mister Negative isn't inherently evil. He's a good person with a dark side that is striving to take over. And players see Li struggle with doing the right thing towards the end of the game, specifically in the cinematic that leads into the boss battle with Spider-Man. Finally beating Mister Negative is only half the fight for Spider-Man, though. The other half is finding the cure for Devil's Breath while simultaneously taking down Doctor Octopus.
Page 2 of 2: The REAL Villain & How Spidey Saves The City
Doctor Octopus - The REAL Villain - Is Born
What really makes Marvel's Spider-Man story work as a whole is Insomniac Games' dedication to the slow burn. From the very start of the game, Peter Parker's professional and personal villains gang up on Spider-Man at the Raft, it's revealed that the person responsible for the prison breakout is really Octavius. It was hinted at earlier in the game that Octavius may have been contracted to upgrade the Raft's security measures, but instead, he used that knowledge to put his megalomaniacal plan in place.
Even though Mister Negative's plan to release the Devil's Breath is thwarted, Doc Ock takes it upon himself to do with Martin Li couldn't: infect New York City with a substance that Osborn created, despite the fact that it can be classified as a biological weapon, with everyone - including his own scientists - telling him that it should be destroyed. But Osborn's sole reason for continuing the project in the first place was to save his son, Harry Osborn, from dying of the same disease that killed his wife - who was the main reason Osborn created the Devil's Breath, to begin with. Martin Li was one of several patients who was being tested for a cure for Norman's wife all those years ago. Octavius didn't want any more part of it after the failed experiment, but since he couldn't strike out on his own, with Norman seemingly curtailing his every move as mayor, Otto decided to take matters into his own hands and bring down Norman for good.
But it's worth noting that the chip that makes Doctor Octopus' suit work is also what affects his mind, effectively turning him evil. That's why he doesn't feel any remorse about what he's doing, in spite of knowing that Peter Parker is really Spider-Man and that he's starting what could become a worldwide epidemic (think: a Planet of the Apes-type of extinction.) While Norman Osborn hasn't actually done anything wrong in the confines of Marvel's Spider-Man story, everything he's done in the past is now coming back to haunt him - and that's partly because he didn't get rid of Devil's Breath when he had the chance. And now, all of his enemies are looking to use that weapon to make him responsible for the deaths of thousands.
Spider-Man Saves The City (And The World)
When Spider-Man discovers and infiltrates Doc Ock's secret lair, he uncovers the real reason the Sinister Six are helping Octavius in his pursuit to destroy Osborn: they are all getting something personal in return, whether it's a cure to an incurable disease or achieving the full potential of one's powers (or even the opposite - taking those powers away). As MJ states, the only way for Spidey to defeat someone as strong as Doctor Octopus would be to dismantle him from within. Since Peter unwittingly helped create Doc Ock, he knows how to defeat him... with science. But first, he creates an Anti-Ock suit that aids him in his efforts.
By this point in the game, Spider-Man's ultimate goal is simple: defeat Doctor Octopus and retrieve the cure for Devil's Breath. They finally come to a head atop Oscorp Tower, where Spider-Man destroys some of Doc Ock's legs and removes the chip from behind his neck. By doing so, he rendered Octavius powerless. And he took the risk of letting the once-good doctor tell the world his secret - revealing his real identity - but silencing him (read: killing him) wasn't something that Peter was willing to do. His innate righteousness is once again put on display when he chooses to save potentially thousands of people with the cure rather than only Aunt May.
Before the game concludes, it fast-forwards a few months to Peter meeting Mary Jane at Mick's, and them not only agreeing to get back together but move in together. That also comes after MJ reveals she got the Associate Editor job at The Daily Bugle. While this is the end of the main story, a sequel to Marvel's Spider-Man is teased throughout the story - with Oscorp retrieving the Devil's Breath for Norman Osborn to continue testing, for instance - as well as in the end-credits scenes. Whether or not Insomniac chooses to move forward with a sequel remains to be seen, but a story like this is why movies like Deadpool 2 work so well: the story is a personal one and the enemy's defeat strikes on an emotional chord with the character, yet the seemingly small victory also ends up saving the world.