I like to think that, because Starlight Glimmer taught the bullies the error of their ways, they befriended Fluttershy and taught her to have confidence, and now she’s a badass.

@Background Pony #2FF7  
That honestly makes a lot of sense.

@Shritistrang  
Here’s a point to ponder. The reason this timeline came to pass was because Starlight actually convinced the bullies to become friends with her.
 
Here she is, having learned she could get along with others, only for Chrysalis to take over. I wouldn’t be surprised if she was so nice and trustworthy that changelings impersonated many ponies she trusted only to be traumatized badly by something they did if she wasn’t so naive in trusting others.
 
The trauma was most likely so severe that she shut off her emotions. Let that sink for a sec.

This is what could have been… if Fluttershy would overcome her shyness and friendliness, if somepony were to push her, force her down a darker path… she would become this! A warrior of the wilderness, a survivor, a master and ally of beasts.

For some reason, reminds me of looking at anthro Red XIII