I created Mysterio to give the world someone to believe in! I control the truth! Mysterio is the truth!
~ Quentin Beck/Mysterio's most famous quote upon confronting Spider-Man.
Tony Stark is gone, there is a window of opportunity, and someone will step up. These days, you can be the smartest guy in the room, the most qualified, and no one cares. Unless you're flying around with a cape or shooting lasers with your hands, no one will even listen. Well... I've got a cape and lasers. With our technology, and with E.D.I.T.H., Mysterio will be the greatest hero on Earth! And then everyone will listen!
~ Mysterio speaking to his crew.
If you were good enough, maybe Tony would still be alive.
~ Mysterio blaming Spider-Man for Tony Stark's death - his infamous quote
He is a former Stark Industries illusions specialist who wants to become the world's biggest superhero in place of the late Tony Stark, feigning attacks from inter-dimensional beings called the Elementals to do so. A spiteful and emotionally unstable man, he ends up becoming the first real personal threat to Peter as his actions have, for the current moment, ruined the boy's life. He is the former friend turned archenemy of Spider-Man.
Unlike the other villains in Spider-Man films who became evil due to certain circumstances, Beck became evil on his own for no reason other than petty spite.
He is far more cruel and heartless than his comic counterpart or any of the Spider-Man film villains in general.
Tony Stark stealing credit for Beck's invention and firing him is a flimsy excuse for his actions because:
He was fired for being unstable. His temper tantrums, sadism, and overall arrogance throughout the film certainly support that claim.
He misremembers the events of Captain America: Civil War, where he imagined the audience laughed when Tony unveiled his glasses' names as B.A.R.F. In reality, the audience didn't laugh at the name, and Tony himself admitted the product needed a much more suitable name, thus presenting Mysterio as a narcissist who has an unreliable grasp on his memories.
He endangers countless innocents that had nothing to do with his job loss at all.
Quentin criticized Tony for using his technology for psychotherapy, despite Quentin himself using this technology for deceptive practices and causing harm (creating the Elementals to make everybody think that he's a superhero).
It is pretty obvious that he felt no gratitude nor sadness over Tony's Sacrifice and death, seeing it as a window of opportunity to take his place as the world's greatest hero.
Staged all of the global destruction caused by the Elementals to pretend to be a hero, completely uncaring about any resultant civilian casualties. He even told his crew to have the drones do maximum damage, and when one of his crew told him that it would cause several casualties, he merely replies that it would serve as more coverage.
Threatens to execute his entire crew with drones over the loss of one of his projectors. Moreover, this loss is actually Quentin's fault.
Tortures Spider-Man by making him go through nightmarish illusions, even blaming him for Tony Stark's death when Peter had nothing to do with it as Tony sacrificed himself to defeat Thanos.
Purposefully led Peter in front of a train using his illusions, which would have killed him hadn't it been for Peter's superhuman strength.
Attempts to murder Peter, MJ, Ned, and Betty in order to keep his villainy secret. It's also worth noting that the latter wasn't even confirmed to know his secret, as Peter said Ned might have told her.
Also attempts to murder Nick Fury and Maria Hill to sell his cover story and prevent them from investigating him, unaware that the Skrulls, Talos, and Soren, are standing in for them.
Feigns respect for Peter as he "hands" E.D.I.T.H. over, only to reveal it was an illusion so he could try to shoot the boy to death from behind his back.
Given that he was threatening to kill his entire crew for losing one of his projectors and that he was constantly yelling at them for either not doing what he says or for making errors, Mysterio never genuinely cared for them and only saw them as puppets so he could become famous.
While it is possible that Mysterio may have had some fondness towards Spider-Man, as he rolled his eyes upon discovering Peter knows his secret and blamed William for killing him, he had no problems with repeatedly tormenting and trying to kill him after he uncovered his fraudulence, thus subverting any redeeming qualities he had.
Planned to destroy most of London and kill thousands of civilians using the hidden drones in the Elementals, all to stage an Avengers-level threat and usurp the position of the next Iron Man.
Frames Spider-Man for killing him and staging his own attacks while revealing his secret identity to the entire world posthumously, thereby achieving his own goals in death. This brands Spider-Man as a criminal wanted by both the authorities and the criminals he fought over the years, threatening his own safety and that of his loved ones. This is all done in spite of Peter's kindness to Mysterio as well.
Exposing Peter's secret identity would seriously affect Peter's personal life, leading to him being arrested for Mysterio's murder and also making Peter seek out Doctor Strange's help to make the world forget he was Spider-Man. This makes him indirectly responsible for all of the events of Spider-Man: No Way Home, with Doctor Strange casting a spell that summoned people from every point in the multiverse who know that the Peter in their world is Spider-Man, which nearly causes the collision of the multiverse. Undoing the collapse of the multiverse and Mysterio's actions of revealing Spider-Man's identity, Peter is now all on his own with nobody (not even the Avengers as well as Happy Hogan and Pepper Potts) to help him, including no tech support.
A part of Venom's symbiote was left in the MCU universe, which could evolve into a future antagonist.
This also basically means that he was entirely responsible for the Raimiverse's Green Goblin, Doctor Octopus, and Sandman, as well as the Webbverse's Lizard and Electro, visiting the MCU, making Mysterio responsible for their respective villainies there, including the murder of MCU May Parker in the hands of the Green Goblin.
Though he does have some comedic moments, his actions are still played seriously.
While many other MCU villains have been more scale in the magnitude of destruction that they cause, Mysterio stands out for his lesser resources, his torture of Peter, and publicly revealing his identity as Spider-Man whilst framing him for his crimes which makes him not only uniquely vile for how he indirectly led to the murder of May Parker by the Green Goblin but also universally vile for indirectly starting the events of Spider-Man: No Way Home and nearly plunging the multiverse into chaos when Spider-Man had led to Doctor Strange botching the Runes of Kauf-Kaul trying to select who to be exempt from the spell, as well as Green Goblin destroying the Machina de Kadavus during the climax which unleashed some multiversal dangers.
Mysterio is the third Pure Evil villain ever featured in a Spider-Man film, the first two being both Carlton Drake and Riot, and following Carnage. However, unlike them, Mysterio is the first Pure Evil to appear in a film with Spider-Man in it. He is the only one in a Spider-Man film to not come from the Sony's Marvel Universe.
Not counting Drake, Riot, and Carnage, Mysterio is the evilest Spider-Man movie villain to date. He willingly became evil, unlike the others who were manipulated or became evil by accidental circumstances. He is implied to have killed thousands of people and even indirectly led to the collapse of the multiverse framing Spider-Man for his death.
Despite Mysterio being Pure Evil, Spider-Man: Far From Home director Jon Watts expressed his opinion that he doesn't see Quentin Beck as a villain and that he appreciates his crew. However, the film explicitly shows that Mysterio doesn't really respect his crew, and Watts' statement may have been in jest, so Mysterio qualifies.