For instance, my "Paladin" concept, started being holier than thou, loyal to the empire, and indeed followed this route for a long time. I despised the Undead, and fought them whenever I saw them, I thought myself above the law, implementing the will of the divines, which supersedes any mortal constraints, and I did so with a fundamentalist slant.
It was only recently that, after destroying a camp, merely because they were stormcloaks, I left and hit the road to meet a farmer who declared he was off to join the stormcloaks. As a Tribune of the Imperial Army, I executed him on the spot. It was then I realised what a monster I had become, there was no trial, no jury. Just my judgement. It bothered me. As I took a fort with my imperial allies, I realised each and every one of these stormcloaks, that I was annihilating.. were also farmers like the stupid simple yokel I had decapitated simply for disagreeing with my point of view.. simple people taken in by a grand statement, without concept of the dangers it truly represented, and the corrupt leader they probably never met, and what was worse, I was enjoying their deaths, even so far as to play with them, and toy with them, to torture them. Stand there laughing as they pointlessly bounced off my shield, before getting bored, smashing the weapon from their hands and decapitating them.
I found that I had lost my taste for the battle, and to some extent, I realised how far I had strayed from my intentions, to do right in the world.. when in reality, I probably brought more evil, sorrow and pain than joy due to how I implemented what I saw as Aedric will. This was a major moment for my character, and changed how I approached the game, trying to give people as much chance to do right themselves. To let them yield, or if at worst, if clearly desperate and deranged, using magic to calm them, and allow me to pray for their souls.
It seemed like a little thing, the senseless murder of a Farmer, because he had rebel tendancies.. but it was more epic in terms of the character than a battle for a city.
What moments changed your character? (Don't forget spoiler tags if needed!)