Fate/Zero: Did Gilgamesh plan to kill Tokiomi from the beginning?

"So was rewatching F/Z and was curious about a few things: Gilgamesh was actually goading Kirei into killing Tokiomi, right? And when Kirei and Gilgamesh met in the room after Kirei was ordered to leave Fuyuki and Kirei told Gil he was gonna kill Tokiomi, Gilgamesh still said something like, "Your teacher's servant is here and is your enemy, why shouldn't I kill you for trying to betray him?" Is this because Gilgamesh's honor/kingly conduct needs a legitimate reason to betray his master or he would feel his honor gets besmirched or something? Or was Gilgamesh just messing around with Kirei and would betray Tokiomi regardless?"

This is a question that a Fate fan asked on Reddit. Here's my response:

That is something that I think there can be multiple interpretations for.

Basically the contract between Gilgamesh and Tokiomi was that as long as Tokiomi supplied him with mana and was loyal to him, he would be Tokiomi's "lion".

One thing, though, is that Gilgamesh has a Noble Phantasm called "Sha Naqba Imuru" ("He Who Saw the Deep", aka, "Omniscience Star") that enables him to see all truths and origins and the future. But he suppresses it most of the time because he doesn't want to use it on "boring mongrels" so he didn't know about Tokiomi's plan.

Gilgamesh's interest in Kirei stemmed from the fact that Kirei was selected by the Grail because he had a wish worthy of it but he did not even know what that wish was. He blindly believed that pleasure was a sin as decreed by his religious faith. Gilgamesh was one who lived for pleasure and believed it to be the root of one's humanity, hence he's greatly perplexed and wanted to see what Kirei - when freed from indoctrination - would do. Observing Kirei's path was a pleasure to him.

Gil did encourage Kirei to take the evil path since "joy in seeing suffering" was the first sign of pleasure he saw in Kirei. However, judging from Gil's rage upon discovering that Tokiomi was planning to betray him all along, I don't think that he had planned to kill Tokiomi from the start. Ruthless as he may be, he values loyalty (that's one of the reasons he spared Waver) and was under the impression that Tokiomi was a loyal vassal. The only reason Gil tolerated him was because he saw him as a subject who followed him and offered mana as tribute. What seriously turned him against Tokiomi was finding out the real purpose of the Servants to the 3 families. So he was looking for a legitimate reason to kill him and wanted Kirei to give him that reason. When Gil asked how Kirei would circumvent Tokiomi's Servant, he was seeing whether he actually had a plan that would entertain him. If Kirei underestimated Gil's power by coming up with a stupid/boring/futile plan, I think Gil would kill him. Tokiomi might not have died if Gilgamesh had not found out about his plan. Kirei's true nature was a convenient way to get rid of him.

What if Kirei had answered with, "Tokiomi is boring. I'm a better Master who can keep you entertained, you should partner with me"? I don't think it would have been that simple with Gil, either.