Why didn't Starlight vaporize Chrysalis with a big magic blast at the end of the S6 finale? She just let her escape.
It's a kids show and she is the only good antagonist they have left. All the other villains are reformed or imprisoned.
Yeah, because showing mercy or offering redemption to a defeated foe is obviously the mark of a fool. Except for when it's done by: Twilight Sparkle Princess Celestia Goku Batman Spider Man Optimus Prime Sonic the Hedgehog Captain Kirk Indiana Jones Superman etc.
Plus, from what I've been able to ascertain, Starlight's mind isn't exactly the most... stable. If she'd out-and-out vaporized Chrysalis, a decent chunk of those present at the scene---the resident Princess of Friendship more than the others---would've gotten on Starlight's case for not offering her so much as a chance to stand down, and that could've gone all kinds of wrong awful quick.