Here's my theory: Zeke had to save Cas's life because he is indeed the solution to undoing the spell that cast out the angels, but he can't have him around because Cas would be able to tell that Zeke is TOTALLY LUCIFER! ... Or might in any case be able to figure out exactly what kind of no good he's up to, since he is another angel and all.
Sam's gonna lose his shit over this all right ....
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Sam's gonna lose his shit over this all right ....