As a famous schizophrenic once said, you either die a hero or you live long enough to see yourself become the villain. He might have just had his face melted off, but he was really onto something.
The old face/heel turn has been a staple of dramatic art media for generations, with cinema and TV only trailing behind pro wrestling in the frequency with which they turn to the trope. After all, it's an easy way of getting relatively cheap reactions. Because there's nothing like watching a good guy being corrupted and turning their back on every pillar of morality they'd previously built themselves on.
Sometimes, it's done to show the hero forced into malevolence against their will (as in Fate Of The Furious), but more interesting is when good guys go through an arc (whether in a franchise or a single movie) that completely flips them over the good/bad divide...
12. Terminator: Genisys
For the first few decades of the Terminator franchise's lifespan, John Connor was basically a Jesus Christ clone. He commanded the loyalty of an army of rebellious survivors and was the key to humanity's salvation in the face of impossible odds.
He also got his own dad laid, which is pretty heroic too, even if it made no sense, since there's no way he could have been conceived in the first place to send him back to impregnate his mother. Anyway...
Looking for the ultimate cheap pop, the Terminator franchise jumped the shark and then went back and turned the shark into a Terminator, by making the literal saviour of mankind the actual big bad.
It was an insult (and an idiotic one that didn't really work, come to that), which Paramount clearly felt so much self-hate about that they just put it in the trailers to stop anyone being sideswiped by it. Or, you know, going to see their movie.