There's something about a character who transitions from being good to being evil that truly captivates an audience. These scenes are often the most important in whichever film they're in, and they are arguably the most pivotal scene in any character's development.
When it comes to isolating a moment in time when a character makes that final step away from being fundamentally good to become fundamentally evil, Hollywood has been somewhat hit or miss. Often these scenes fail to meet the expectations of the audience, or bad storytelling coupled with poor cinematography and hammy acting go a long way in making these scenes less of a "wow" moment and more of a "meh" one.
That being said, there are certainly one or two amazing scenes of this type every hundred or so films, and the ones that got it right, stand out above the rest.
Without going too far back in Hollywood history, there are several examples of character transitions that are far better than others. The scenes on this list are those that truly define how a character can become evil after spending their whole life (and their time in front of the camera) as one of the good guys.
10. Kylo Ren - Star Wars: Episode VIII - The Last Jedi
When audiences first met Kylo Ren, he was clearly a bad guy, and it would take another movie to further flesh out how he went from being the son of Princess Leia and Han Solo, and a disciple of Luke Skywalker to the right-hand man of the leader of the First Order.
Ren's transition to the Dark Side was actually spread across two scenes, with one shown from three perspectives. The first scene was canonically his final transition to the Dark Side, having occurred later in his life. It was that moment when he fully embraced his destiny by thrusting his lightsaber through his father's chest.
While that did send him over the edge in terms of which side of the Force he was on, it wasn't the moment that set him on that path. In Star Wars: Episode VIII - The Last Jedi, Ren and Luke explain how he went from a trusting padawan to the leader of the Knights of Ren.
The scene was explained three different times from multiple perspectives, but they all told the same story: one evening when Luke sensed the evil growing in Ben, he went into his home and stood over him with his lightsaber, ready, yet wavering to strike. Upon awakening, Ben saw what was happening, and he pulled the building down around them before heading out to recruit some, and kill the remaining students who sided with Luke.