Controversial take, especially since Twilight's my favourite character, but... · 10:26pm Jul 16th, 2022
I bet Twilight would have made for an amazing villain.
Obviously, not a cartoonish, moustache-twirling villain that belts out an evil laugh every now and then but rather a villain that thinks what they're doing is right and therefore would do anything to achieve their goals. So, we know she had been groomed by Celestia to be the Princess of Friendship and that she would inevitably be Celestia's successor, right? And we know she and her friends have absolutely no problem obliterating or at least petrifying their enemies if the situation calls for it (not that their enemies didn't have it coming). And we know she'd have absolutely no problem bending the rules if the situation calls for it, as this clip shows:
Okay, Starlight might have egged her on into that decision in that clip but still, rules are rules and just because you're in a position of power doesn't mean you are above the rules. That would make you no different from a dictator. Especially if you decide to change the rules because you think it's the best way to spread friendship, possibly at the expense of other cultures and their way of life.
Oh, and who could ever forget the Smarty-Pants incident?
Which means, if Twilight wanted to, she could easily dominate the minds of everypony she saw fit in the name of friendship. And I know Celestia is egging her on into those kind of behaviours because, for lack of a better explanation, she is like the Palpatine to Twilight's Anakin.
Though, of course, this could all be psychological projection on my part, especially since the reason why I like Twilight is because she reminds me of me when I was younger (and because peeps like to diagnose her as an Asperger's/OCPD combo while I have the "luck" of possibly being an autism*/ASPD/Bipolar combo) and one reason why I dislike Starlight Glimmer is because she dropped her goals simply because Twilight gave her a pep talk, which leads me to believe Twilight went all Celebrimbor on her flank. Especially if you believe Friendship is Magic is an in-universe show the same way Buzz Lightyear of Star Command is an in-universe show within the To Story universe even though Lightyear exists in that same universe and...well...need I explain it comes with its own contradictions? 
Anyway, I am way to inebriated to explain myself properly so leave a comment if you want me to explain myself when I'm more sober-minded.
* While I actually am diagnosed with autism, ASPD and Bipolar disorder are merely conjecture on my part because I'm half-convinced I inherited my mum's bipolar and because I also exhibit traits associated with ASPD, which is weird because it's rare for people with autism to have ASPD.
She could be described as a 'gentle' tyrant, as the ruler of Equestria. Because, like you say, she'd do whatever it takes to ensure Friendship succeeds, even if the steps to get these involve things villains would do
So yeah, she could completely flip one day and everyone would just go along with it, bcuz 'FrIeNdShIp'