What single moment ruined a character?

Basically moments like in the Spongebob episode “The Card” where Patrick revealed that his stupidity was a facade that hid an evil mastermind all along.

The episode where Skinner wasn’t the real Skinner.  
Not only ruined Seymour’s character, but the episode was hated by everyone, even the voice actors.  
It was so bad, its was the first episode to be considered non-canon, and was the peak of the downfall of The Simpsons.

Shadow when he came back to life

@Whomps  
https://m.youtube.com/watch?v=aTjsnTvWFuY

That part in Persona 4 where Yosuke tries to kill your only lead.

Discord after attempting to kill the students just because he wasn’t put in charge of the school.

Mr. Krabs in One Coarse Meal

when Adam West died…

It seems Timber hiding behind Twilight shattered what little tolerance people had for him.

@Questionmarktarius  
https://www.youtube.com/watch?v=ZNhWHlaTwOM  
Why ya gotta remind me of that?

Pinkie Pie in Yakiky Sax.

IMO Applejack in applejack’s day off.

Dewey throwing a massive tantrum and leading the pirates to take his family hostage which nearly led to them getting killed in “Sky Pirates in the Sky.”

Not very many for me, but everything Starlight did just being because a boy dumped her a dozen or so years ago (and her being let off scot-free because of it) and Spitfire being a complete Zapp Brannigan in Wonderbolts Academy basically ruined those two for me for good :/

Oh, and Kyle from South Park after “Douche and Turd,” where he tried to get P. Duffy to kill Stan and got him exiled from town all because Stan wasn’t voting for the mascot he wanted.
 
Far as I’m concerned, any and all hatred Cartman directs toward him is entirely justified.