Ah, Extra Credits. I knew it would be that exact video just from the thread title. I actually commented on that one on Youtube, and since it was a City of Heroes example, I'll paste it here as well.

My Youtube Comment wrote:

The one that stands out for me would be my old villain in City of Heroes. The ninja-assassin for hire "White Lie".

I made her as a gameplay conceit that a lot of the quests were mercenary work, so a villain-for-hire seemed like a good fit. Then the update with the alien invasion happened, and of course she'd sign up to fight those, Earth is her home, too, and the Vanguard defense force paid very well for dead Rikti, so off she went to do the new content with a marginal roleplaying excuse.

One of those quests had you save a man for no particular reward, except you got a grateful letter from his mother if you did. Described as handwritten with a shaky hand, and a few smudges due to tears dripping onto the page. And that... was a turning point. I thought long and hard about how she would react to that, other than keeping on taking contracts against the aliens, of course, but when the Going Rogue expansion came out, there really wasn't any question anymore.

White Lie ended up as the only character I ever changed sides with. Not fully Hero, an assassin gotta make a living, but a dashing Rogue, working mostly for the good guys, or at least the kind of good guys in the Vanguard group. All traced back to that one time she did a good deed, and didn't know what to feel about it when called on it by a grateful mother. I think this is one of the few times I've had a character I made and played make a decision "on its own" so to speak. Amusingly, it was an inverse of the one evil act dooms you forever story. It was one good act that slowly took root and made her question her true motives. And all this from a descriptive flavor text on an end-of-story souvenir.

Sometimes it's the little things.