It's Crowley. He'll damn well try.

[Because Crowley's fatal flaw is his pride. Gabriel's got cowardice. Castiel's got heart. And meanwhile, Crowley spends so much time hating Lucifer that he doesn't realize that he's just like him. He'll find some way to do it without Castiel or he'll find a way to coerce him.]

He'll find a way. Or he'll start killing everyone you like until you break down. Toss-up.