When you kill Nazi Zombie Chef, after Clyde burns him when he comes to his senses.
Defeat his "Hello Children" attack, and he'll spend a whole turn with a Thousand-Yard Stare and a horrified expression.
In a way, Butters and all of the places he mentioned he got beat up or bullied in some shape or form and just says it so casually.
During the boss fight against Butters if you choose to side with Kyle's elves, delivering the final blow has him depressingly say that nobody likes him.
On top of that, the description for Butters' hammer states: "You struck down the first friend you ever made in South Park. Then again, it was Butters." That doesn't exactly help much, depending on if you found it funny or not.
Butters even cries when you defeat him. It's heartbreaking.
Especially after he takes his already adorable Apologetic Attacker attitude to the next level by assuring you throughout the fight that no matter what happens, it's just a game and you're still friends out of it.
Someone created a parody of this scene and Ryuuko's brainwashing from Kill la Kill. He might have actually made it worse.