well, 1,000 years ago celestia didn't know what to do with luna, and didn't know how to help her. so she sent her to the moon
same with discord. but.....celestia, and luna most likely came from a neighbouring country, who feared that discord might spread to different countries, so they were tasked to stop him
both of these scenarios, they didn't know any other way to stop/help nightmare moon/luna, or discord
since twilight is the princess of magic/friendship, it's easier for her to help redeem villains, as long she she thinks they can be redeemed
with starlight, she never wanted to destroy equestria, she just wanted to stop twilight and her friends from ever getting their cutie marks
and once twilight has shown what happens if none of them gets their cutie marks. she does feel remorse, and guilty about it
if she didn't feel remorse, or guilt about it, then i do believe twilight would've stopped her another way