I'm almost certain that I've witnessed a fanfic writer's wet dream.
I've seen cases of Back to the Future syndrome before, but good lord this was one of the most extreme. So many alternate timelines, all showing what happens after the villains won. Wars, fights for survival, complete desolation. Christ, there's just too many sad emotions just thinking about 'em.
Starlight's backstory doesn't justify anything she's done, but then again, we've seen people who've taken extreme measures to prevent their past from happening again. (Hell, I even know a couple folks who do just that). She just didn't realize what the consequences of her actions were until someone like Twilight took a stand and showed her. It took a lot more than just showing a decimated Equestria to convince her to try another way, but I'm impressed Twlight managed to do her job as the Princess of Friendship and let Starlight have a second chance.
And y'know what, I'm glad to see Twi take such a powerful unicorn under her wing. It hearkens back to when she was Celestia's student, just a little filly with a lot of power just waiting to be released. Now she's got a fully grown unicorn to watch over and make sure that power is used for everyone's benefit. I look forward to seeing what these two will get into next season!
It was more "You have to live with your past, but you can't let it define who you are" to me. Simply going back in time to change the past doesn't help you learn what went wrong. It's more a foolhardy bargaining tactic that more than likely will make things worse.
This was a fantastic finale, easily the best one they've done. Here's to Season 6 and the movie!