And I don't mean Fallen Heroes (as in heroes who deliberately became villains), but I mean villains who came close to becoming villains before joining the heroes side. And this discussion indeed revolves heroes who came close to becoming villains, but without actually becoming official villains (and no, anti-heroes do not count for that reason, and neither are anti-villains). If you have any heroes that almost came close to becoming villains, feel free to share.
For example; Sheriff Woody from Toy Story was a hero yet became a pessimistic and snarky individual when Buzz Lightyear arrived as part of Andy's birthday present. He felt as if Buzz was stealing his spotlight, even mentioned that he'd love to see Buzz as a crater, further cementing his obvious jealousy towards him because he deserves more attention to his owner Andy than Buzz.
And even before that, he was calling out on Buzz because he feels as if Woody is being left out in favor for a much better toy so basically, he almost instigated a fight between himself and Buzz. What led to nearly cementing his status as a villain was when he controlled RC and tried to knock Buzz out the window and tried to make it look like an accident. Not only was he a terrible liar, his friends knew that he was trying to get rid of Buzz so that he can be played more by Andy so they almost beat him to a pulp, had it not been for Andy entering the room.
While he did eventually felt resentful, he did went back to his habits in Toy Story 2 by almost siding with Stinky Pete in joining with the Roundup Gang in the museum in Tokyo, which would have permanent consequences had he joined up with him, as museums have heavy surveillance, and that would mean he, Stinky Pete, Jesse and Bullseye would be unable to move whatsoever due to being watched by security cameras.
And in Toy Story 4, he was desperate to join up with Bonnie so bad, that he doesn't realize that Bonnie wasn't even remotely interested with Woody at all. And because Bonnie doesn't share the same bond Andy has, Woody realized that he's pretty much better off without an owner and simply be with Bo Peep. Sure he'd have to say good-bye to his friends, especially Buzz Lightyear, but, it was for the better.