The Oathbreaker NPC is a perfect case in point of a "Good" Paladin who breaks their oath but doesn't become evil. In my 5e D&D campaign I'm playing an Oath of the Crown Paladin who follows a Lord because they believe in the order they bring to the region. If that Lord, just as the one on the NPC's story, changes his focus and works contrary to the intent of my oath I would be happy breaking it to become an Oathbreaker. Technically I have forsaken my vows, but I haven't done it for evil reasons.
I would like to see Oathbreaker be a legitimate subclass, they have different skillsets and abilities although they do seem to be more evil oriented. They can still be played as Good, using those powers to promote the Light while stepping into the darkness now and then.
