Heheh as someone whose written extensive rewrites of Sunset’s life and backstory, I can say for sure I have my own take of it.

I like to think of Sunset’s descent into evil as a negative feedback loop. Here’s a graph

These two set each other up for failure without even knowing. Sunset wanted a teacher but needed a mother. Celestia needed a student but wanted a daughter. They fucked themselves UP.

The loop continues! Sunset overachieves and pushes herself. She makes mistakes, but Celestia forgives them, becaus ehse sees the best in Sunset, and doesn’t ever want to hurt her.

Add a dash of exterior forces saying Sunset is just a charity case, the spice of Ngihtmare Moon’s imminent return making Celestia think Sunset is prophecized to be the one to beat her sister, and well… A very avoidable ending can be hit like a brick wall.

For more information, go read Forgotten Sun.