Originally Posted by
WhateverIsntTaken
Well, that shows Caster was aware that someone was behind the whole mess, but not much more than that. I mean, personally I find it more likely that Hassan killed Kuzuki, Caster maybe slashed him (Hassan) with Rule Breaker without managing to break his contract, and then Hassan disappeared right on cue for Saber and Shirou to walk in and jump to conclusions. (Unless there's some element that explicitly suggests that Caster stabbed Kuzuki?)
Or what happened in Chaos Theory.
I find most things most likely than Zouken out-magecrafting Medea of Colchis, really. He's good, but nowhere near Age of the Gods good.