I've heard a lot of people bash Michael, saying his "teaming up" with the monsters is absurd, but it's really not. I'll explain.
Consider his actions in Apocalypse World. He tried to create a world he considered perfect by purging those he deemed unworthy, but he eventually realized he'd made a mistake.
In Sam and Dean's universe, he's trying to do something similar, but different. He wants to find beings he considers worth saving and create their version of a perfect world.
With the humans, he saw that they only told him what they believed he wanted to hear. They're cowards and liars. They're not worth saving.
The angels - the few that are left, that is - are broken, fallen, flawed, human. They're not worth saving.
The demons are not worth saving, for obvious reasons.
The monsters, however, are a different breed altogether. Monsters don't pretend to want peace on Earth. They don't pretend to care about the suffering of the people they eat. They don't pretend they want anything. They only want one thing - to eat - and they don't pretend otherwise. They're pure, and thus worth saving.