Young scars can go deep and they can fester a long time.

I see people calling this a weak motivation. While I certainly find my headcanon motivation for Starlight stronger, this actually sounds like a pretty sound motivation to me. She had a friendship torn apart because a cutie mark (or rather the attendant responsibilities that came along with said friend’s mark) got between them. She became bitter, angry, and resentful. She tried to channel that anger and bitterness into something she thought was a constructive pursuit: making sure that no other pony had to suffer the same pain she did. Of course, she didn’t realize just how delusional such an idea is. And when she was thwarted, she had no more “constructive” outlets for her rage and fell back on plain vengeance. It took seeing just how much damge that pursuit could cause to snap her back to reality and admit that she’s been stuck as that sad, angry, scared filly for who knows how many years? 20? 30?

Her Energy Weapons stat is pretty good too.

@Lupin Quill  

 
+1, best comment ever!

@Lupin Quill  
She’ll do fine in gathering many allies in the Fallout Wastelands.

at this point, no one can doubt that Twilight has 10 charisma, level 100 speech and is doing pretty fantastic on her pacifist run.