Mysterio being a villain was a twist???

You just described “character motivation”.

When a superhero is revealed to be a supervillain after a substantial amount of a film’s running time has passed, that is a “plot twist”.

In this case, a totally transparent one.

Unless you’re saying that it was intended to be obvious that Mysterio was a villain from his very first appearance in the film?

Oh, it was meant to be a twist. I "spoiled" the twist to many people on Facebook and also to my alliance in a mobile phone game I play the moment people started posting on Twitter from early screenings that Mysterio was the movie's main villain. I got a lot of backlash for that. Half my alliance members hate me now for spoiling the twist and had requested to the leader and officers to kick me from the alliance, and a lot of people said on Facebook when replying to my comment that they can't see the movie now because I spoiled the movie for them by simply confirming that Mysterio was/is the main villain of Far From Home. There were a lot of people convinced that Mysterio WAS from another universe and the version of Mysterio that came from another universe was a good guy. You try telling them that Mysterio is one of Spider-Man's well known villains and you'd get "no, no, no, the trailers and TV spots said he is a good guy, so this version of Mysterio is a good guy. No way is a bad guy. He can't be. Not possible." But all I spoiled was Mysterio was the main villain. Never went into detail about him being someone that worked for Tony Stark and got fired and helped create BARF and all that. From people on Facebook, I received TONS of death threats both in comment section replies and private messages through Messenger, and I was called all kinds of names. So a lot of people were convinced Mysterio was a good guy and WOULD be a good guy through the entire movie and that the actual villain version of Mysterio would either be revealed in the movie during the final battle, or maybe in Spider-Man 3 or some other MCU movie that Spider-Man pops up in.

If Quinten Beck worked for Tony Stark, how didn’t Shield have a file on him?

No movie should assume the audience read the original source material

"I wouldn’t say it was played like a big shocking twist, it was just a reveal. A really casual fan might be surprised by it but it doesn’t really matter either way, the point was for Mysterio to trick Peter Parker not for the movie to trick the audience. It’s more one of those ‘I knew it!’ kind of reveals rather than ‘I did not see that coming’ "

Yeah, I'd go with this.

I suspect that most people watching the movie know that Mysterio is a villain. My feeling watching the movie was pretty much what you described.

so Yes you gigantic IDIOT

I prefered that he really worked in cinema like in the comics.