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.