I’m not going to make a lot of friends with this, but I think Starlight is a lot better written than Tempest is and I know it sounds unfair to compare a character who’s been around for four seasons to someone who made one movie appearance but since there is some debate out there with Tempest being seen as better written, I have no choice but to believe this is something that can be compared.

First, we should start with their reasons to turn evil. Tempest turned to the dark side because she broke her horn. Starlight turned evil because her best friend got a cutie mark and was sent away because of it. Neither are necessarily the best backstories in the world, but I feel Starlight's is a bit better. Before you all mount my head I can explain, with Starlight’s backstory, it’s implied that things happened out of her control and she slowly grew up with the idea to take cutie marks away to make everypony equal since she clearly went through some phases in her youth(the implied goth phase in Parent Map for example), and no one likely ever talked her down until she met Twilight.

Meanwhile, Tempest broke her horn when she was attacked by an Ursa Major after ignoring some very obvious signs warning ponies to stay out of the cave. Everything that happened to Tempest was entirely her own fault. And before you rage in the comments, look at the song number again, there were warning signs and the cave was boarded up. Even kids can follow simple directions.

Next we should look at how they reformed. Starlight had her entire belief system questioned by Twilight and after a very long battle finally saw the error of her ways while Tempest was oh so shockingly betrayed by a warlord.

The reason Starlight’s reformation worked is that she had to see what her actions would cause and Twilight was able to tune into Starlight’s emotions with genuine sympathy and even then, Starlight had to think before taking her hoof, heck there were TEARS in her eyes. She clearly had to reflect on her choice. Starlight was broken, not evil.

Whereas Tempest really only reformed because the Storm King betrayed her. It didn't even really feel like a real reformation, just Tempest just getting scrappy seconds because she didn’t get her way.

Quick question. Was Starlight written at ALL?

Warning, LOTS of swearing...

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I have linked a video, that explains Starlight's absence of change. Now I want to go point by point, and directly say what things you have gotten wrong or right.
1) Actually, Tempest had a better reason. She lost a part of her body, something that DEFINED a UNICORN. She was a UNICORN, now she became an earth pony with ZERO affinity. No plants, no animals, no strength. Whatever she HAS studied(levitation/light/whatever) became USELESS, as she did(does) not have a functioning horn. She wasn't just dropped by her friends. She was dropped by her TWO(2) friends because she wasn't safe to be AROUND. Whether it was out of fear of her, or pity when Fizzle could not do things - I don't know. Imagine you are born as a magician. You can levitate, make light appear, whatever. It's who you ARE. Now, imagine that piece of your SOUL being RIPPED away. You are just a regular mortal. And you see other magicians GO and DO stuff that you USED to do, but you can NOT anymore. Not without arcs of lightning, threatening health, life, and just general damage ot objects around you.

Warning sings and boarded entrance. Are you joking right now? Are you KIDDING? Kids will go where it is FORBBIDEN. just to see WHAT HAPPENS. Are you a MORON? Kids, when said not to do stuff, will DO stuff.

You mean, boarded VISUALLY? Kids were playing BALL, they didn't even SEE the boards. Granted, how would the Ursa come IN and OUT with those boards in the way is another question. Nevermind that. Boards are WAY too hight, kids didn't even SEE them, when chasing ball.

Starlight lost ONE friend, when she got a freaking CUITIE mark! She didn't HAVE any OTHER friends? She didn't TRY to write him, EVER? She just though, "cutie marks = bad, need to rid the world of this thing". This is your idea of a good villain backstory?!

2) Let me spell it out for you. Starlight was BARELY TALKED OUT, and didn't learn a THING. She is still the OLD Starlight, that goes for the blood, whenever something doesn't work in HER favor. Tempest was a KID when she met Storm King, of COURSE she would know nothing of betrayal. Nevermind that Tempest is a PONY, and they GENERALLY don't do betrayal(mentally unstable monsters like Cozy aside)... You have someone who trusts, and puts a lot of loyalty to someone, until that someone very deliberatly breaks it.