People don't release how much Wills bullying hurt him, truly hurt him. And how deep ingrained the idea he's a mistake is.

Will grew up with Lonnie byers as a dad. Lonnie tried to force Will to like things he never would, like baseball, because all the other-the normal-boys liked it. He called Will things such as 'fag' and 'queer'. While this might have only been something he told to Joyce its clear Will knew his farther thought he was different.

Will has been bullied all through school for his interests to his choice of cloths. We see Troy calling Will a fairy and a bunch of other stuff even when the whole town thought he was dead, that Will, and 11 year old boy, was murdered because people thought he was gay. Bullys build off eachother so the amount of bullying Will must have went through to make the bullys think that was OK is insane. In season 2 Will gets a nickname, one other then fag, fairy or queer, he gets called zombie boy. I personally think this was probably the most hurtful name he was called especially after everything he suffered in the upsidedown. Zombies are only half human, they are disgusting, dangerous and infectious-sound familiar? 1983 was almost ot the hight of the aids epidemic, gay men were labelled these things to further dehumanise and vilinise them just like what the kids (and probably some adults too) treated Will like.

Coming to the realisation you're gay is scary but in the 80s is was downright terrifing, knowing that if anyone evan had the idea you're queer could get you killed. But Will basically lived that. He came back into the world after everyone thought he was murdered via hatecrime and hardly anyone cared. Will got a taste of what could happen if he was hatecrimed, the fact that rumours and bullys were enough to make a town disregard him. And the worst part is (for Will) they're right. The bullys were right. Because he is gay and there's nothing he can do to change that, all the bullying and name calling, all the side eyed stares in the locker rooms and parents whispering to eachother about "the byers queer kid" , all the times he was thought of as a mistake was right.

That's why it hurt so much when Mike said 'it's not my fault you don't like girls!' Because Mike never treated Will like he every one else despite if it was true or not. He was Wills safe space where Will could be his funny, nerdy, happy self free of judgment-until that day. With that one sentence Mike confirmed all of Wills fears , he knew Will was different, he knew it had something to do with him and he did judge Will for it. Of course as the audience we know this isn't really what Mike thinks but Will doesn't know that.

The destroying of castle byers represents Will tearing down his comfort in himself with hatered. Even the name castle byers puts Will in a position of power, it's not hard to imagine young Will pretending to be king of the castle: there's no king anymore. All his memories from there, Jon helping him build it, his mum messing with him, the sword leaning against its entrance almost definitely left by a certain paladin, flattened on the wet ground as he physically tore his happiness and comfort in himself and feelings for Mike down. Because its stupid, it's 'so stupid' because his feelings are wrong. Because stupid things are mistakes.

Will has always felt like a mistake.