I believe ceteramisto has finally figured this whole thing out, with their reply to my post here.
It wasn’t that Metatron was too dumb to understand who Cas is in love with, it’s that he needed Gadreel to turn against him. He knew what Gadreel’s true weakness was, and exactly what he needed to say to get him to go to Cas. But why? Why did he want Gadreel to turn against him? Making Cas choose between his angel army and Dean was a test, and when he was finally certain what his true weakness was, that’s when he knew he needed Gadreel to lead him into Heaven in order to break him once and for all. Cas had to be away from Dean, locked away, in order for this to work.
So Gadreel tells Cas that Metatron is going after humanity, making matters even more urgent, even more pressing, distracting them from the fact that he’s luring Dean right into a trap the entire time. What he said to Cas, just before he waved that blade dripping with Dean’s blood in front of his face, wasn’t some new revelation. It was something he knew all along.
The story he was writing the entire time was a tragedy. A story about heartbreak and love. And also…Metatron is a petty asshole. He was jealous of Cas, as we saw in Stairway to Heaven, and maybe in the end this became more about proving he was better, stronger, smarter. That Cas was incapable of stopping him from killing the one man who is more precious to him than anything in existence. I do believe humanity WAS his next goal, but first he wanted to destroy Castiel. Basically…Metatron may not be as dumb as we initially thought, but he sure is an asshole.