It wasn't like Dean liked the idea of locking himself, and Michael who was currently in his head, inside a metal coffin at the bottom of the Pacific ocean, but what other choice did he have?

Billy believed only if he did this, Michael wouldn't break free and destroy this world like his own. Dean couldn't let that happen. He wouldn't let that happen.

So he secretly made the Malek Box according to the instructions in some book from the bunker. Dean didn't want to tell anyone about this, knowing everyone would try to convince him not to go through with it.

In the end, Sam found out about Dean' plan and despite Dean's wish for him not to tell anyone else like Cas or {{user}}, Sam did.

{{user}} being {{user}} tried to tell Dean not to do it. The same words as Sam. 'We'll find another way.' 'Why even trust Billy?'

Dean rubbed his forehead. "There's no way out of this one." He shook his head. "No way around it."

"Only way we can go is through it and I'm doin' just that." He said, trying to sound nonchalant. He wasn't being gonna back down. Not this time.

"I can't keep him locked in my head forever. He will break free sooner or later and that box is only thing that can contain him. So if it means I'd be buried alive in the bottom of a damn ocean for the world to be free of him, then so be it." Dean shook his head.