OK, this is what I think about Zeke...
The angel who called himself Ezekiel and convinced Dean to collude with him and get Sam's agreement for Zeke to possess Sam as a vessel... well, I don't think he's the same angel that Castiel knew and vouched for.
He may or may not be a bad angel, but I reckon he's a different angel. That would provide a couple of good reasons why he doesn't want Cas around.
Although Cas hasn't recognised that Sam has a passenger yet, he presumably might and also Zeke might be worried that Dean could confide in Cas. Because Dean obviously feels guilty about the whole shebang and while he's too scared to tell Sam and risk Sam dying because of it, he might (in a moment of weakness) think that Cas would understand his dilemma and be a sympathetic ear.
And if Dean did tell Cas about Zeke's whereabouts, then Cas might question him further and discover that this angel is not the one he remembers fondly as "a good soldier".
It would seem that, Ezekiel or not, this angel doesn't wish harm to Castiel per se, but he also is unwilling to let Castiel's problems put himself at risk.
I think he's an interesting character. I just don't think he's really "Ezekiel".
Poor Cas, though! That last shot of his face when he's told he has to leave was heartbreaking. You know he won't kick up a fuss because he's still guilt-tripping and any suggestion that he might be endangering Dean and Sam would get him going into full on martyr-mode. And that kinda makes it even worse. He didn't hold hand out asking for help, but after suffering all on his ownsome, got rescued by his friends and he seemed so damned happy and grateful. And now to have that rug pulled out from under him... Poor, poor lonely angel boy.
And look at this. First random LJ post for the year. What? It's only October.