Will Byers doesn't hate himself, he is just deeply traumatized but he is slowly healing with the love of his new family and finally getting over Mike.
Mike Wheeler hates himself quite a bit, he knows he's gay and knows that there's a possibility that Will loves him, but after Will encouraged him to say 'I love you' to El, he thinks he has really little chances with him.
Mike tells Will how he feels after collecting enough data from Will's reactions and actions and also to not get possessed by Vecna (the group has to tell each other their secrets so that they have less possibilities of getting possessed ). Will starts crying. Byler kiss.
I wanna explain a whole ass Lumax theory that I have in my head, but I'll leave that for later.
To kill Naruto would be killing myself. It’s an endless cycle of hatred that we were born into; there is no choice in this. We weren’t asked how we felt, or if we even wanted this. No. To kill each other is our birthright. To be lonely forevermore once our other half is slaughtered like a lamb (or a mangy wolf, what is there that is innocent about me anymore? I killed that child long ago, when I left him. Or maybe it was Itachi, Konoha, or anybody else, maybe I wasn’t to blame.) with our own very hands. Bloodied and broken, a heart torn in half — decaying at the seams — as why would I live if I killed him? Death was our gift from the Sage. From the beginning to our end, we’ll take the sword and plunge it into our very hearts which we know better than anyone else.
To kill Naruto would be killing myself. So why not die together and end it with each other, for the final time? Will you grant me one last selfish request, Naruto? (Wasn’t it your idea, my one and only?)