Nah, Starlight may not exactly be good for now. But she’s not as cruel-minded or quite as fueled by hate as Zathir is.

Note she never actually hypnotized anybody in the Season 5 opener. The way they creepily always smiled and were content with their way of life is because Starlight manipulated them into believing that’s how life should really be. While they still acted quite creepy, the Ponies of “Our Town” were acting on their own volition. As soon as the town saw that she was lying about her cutie mark the whole time, they revolted and demanded their Cutie Marks back.

Zathir’s method straight up required a really strong hypnotic device to reach his goals. And he wouldn’t bat an eye for anyone effected by the device or the casualties that would come from anypony who tried to stop him

((WARNING: MAJOR POSSIBLE SPOILERS FOR SOMETHING IN THE SEASON 5 FINALE BELOW THE BREAK. DO NOT CLICK IF YOU’RE SENSITIVE ON SPOILERS))

Besides, it sounds like Starlight’s going to be reformed by the end of the Season anyway. The Season 5 poster has Twilight looking at Starlight’s silhouette as if to indicate she’s smiling to a new friend, and there’s a book that implies she becomes a student of sort for Twilight.

Zathir on the other hand is irredeemable. he’d rather die then admit he was wrong to do anything he did.