Warning: full spoilers for Spider-Man: Far From Home ahead!
When Mysterio was portrayed as a potential heroic mentor in the initial marketing for Spider-Man: Far From Home, fans who knew their comics were already aware that something was amiss. After all, Quentin Beck’s M.O. in the comics is being a master of deception, using illusions, hypnosis and plain old sleight-of-hand to manipulate his adversaries into seeing what he wants them to see. Beck not quite being what he presented himself as was expected, but what perhaps wasn’t expected is exactly why his scheme was so effective at conning Peter Parker.
Specifically, Mysterio exploited both the in-universe and audience expectation of world-ending large scale conflicts that have become commonplace in the Marvel Cinematic Universe to enshroud a scheme with ramifications that are far smaller and intimate. By not being a global threat, Mysterio arguably becomes a far more potent one, and this is what allows him to have such a monumental impact on Peter’s life.
Source: IGN.com Spider-Man: Explaining Mysterio's Metatextual Commentary