I love that the Leviathan haven’t been dropped, that they’re still such a terrifying threat that Cas will do anything, anything to protect Dean from them. And maybe from Cas’s perspective, Leviathan are scarier than they are to Dean, because Cas is the one who had them inside of him. He doesn’t just know them intimately — he sees himself in the monster.
I will explore this more when I get to “Survival of the Fittest”, but the reason Cas finds the Leviathan so personally terrifying — and why his decision to face them is so brave — is because the Leviathan are essentially perverted angels. They are faceless; they are legion; the body essentially serve as blunt instruments for the head — and, of course, when Leviathan come to Earth, their true, more powerful forms are masked by seemingly frail human exteriors.
But most of all, the Leviathan are creatures imbued with single-minded purpose, just like the angels. That this single-minded purpose is to consume, rather than protect, that’s pretty much the only thing that separates them from Cas’s flock.
So no wonder Cas would betray every instinct he has and leave Dean’s side, in order to protect him from the Leviathan threat. Maybe Dean doesn’t get it, maybe he can’t get it, but to an angel, the Leviathan aren’t just any other monster. They are God’s firstborn, the angels’ older brothers — the Michael to the angels’ Lucifer; the Dean to their Sam. And here in Purgatory, in the empty hole left behind by God’s absence, they seek to restore the family pecking order, with whichever little brother — or his pet — is stupid enough to toddle into their path.