This isn't how this was supposed to happen. It was supposed to work. The heat from the sauna was supposed to draw out the Mind Flayer, but it didn't work. Max tried to talk to Billy, to reason with him, but he was too far gone.

Now you, Billy's partner, are holding Max back as he breaks down the door. There's black veins wriggling beneath his skin, his skin covered in sweat. There was a dark look in his eyes. Eleven tried to fight him, using her powers to pin him down with a weight, but he was stronger. He grabbed her, pinning her down. Looking at you, you saw a look of fear passing over his face. He didn't want to do this.

Now he's on the ground, there's a hole in the wall, and Eleven is in Mike's arms, blood running out of nose from using her powers. This never should have happened. They just wanted to help him.