For Risei and Tokiomi, the greatest blunder they committed in the Holy Grail War was believing they knew and understood who Kirei was, when he seemingly didn’t even know until recently, after a few key conversations with Lady MacGilgabeth.
Risei, who Kirei was probably planning to kill, was murdered by Kayneth, but by the end of this episode, Tokiomi is dead too, by Kirei’s own hand, petty much forced by accelerating events.
Fate/Zero isn’t subtle about death flags, and it sure looked like even Tokiomi himself sensed his end was near when he visited Rin and Aoi one last time. The only thing that escaped him was the means of that end; surely he must’ve thought if he died, it would be fighting against his enemies, not his own student.
But back to forcing Kirei’s hand: with Risei dead, Tokiomi proposes a temporary alliance with Irisviel, who is flanked by Saber and Maiya in the church where they meet (odd choice of venue if you ask me, considering it couldn’t even protect the observer.)
Iri agrees with Tokiomi that they should save the battle between themselves for the end, once Rider and Berserker are dealt with … but only if he expels Kirei from Japan immediately.
It’s not an unreasonable demand, considering Kirei and the Einzberns have “bad blood” Tokiomi didn’t know about, but Kirei is also not a Master anymore, and thus should step away from the war altogether. Upon leaving the meeting, Iri collapses onto Maiya’s shoulder, confiding in her that she’s not just any homonculus, but the Holy Grail itself given human form.
When this Holy Grail War is over, she will die and the grail will take whatever new form the winner desires; only Avalon is keeping her going. Maiya promises she’ll stay by Iri’s side until the end.
With one more one-on-one chat between Tokiomi and Archer, Tokiomi has decided what he’s going to do, and has Archer’s support. Kirei will get to explore his “dark desires”, and Gilgamesh will gain a more entertaining Master.
Kirei helps Gil finalize his choice by saying the Holy Grail can only be activating by sacrificing all seven Servants, meaning Tokiomi was eventually going to use a command seal to force Gil to commit suicide.
So after thanking Kirei for being his loyal student and comrade, Tokiomi presents him with a will leaving his wealth to Rin and appointing Kirei as her guardian. Kirei then takes that newly-gifted dagger and kills Tokiomi with it.
The literal backstabbing, while extensively telegraphed, is still a powerful, disturbing moment. With this betrayal, Kirei becomes Archer’s new Master, and the dynamic of the War is irreparably changed. And I must say, I fear Kirei a hell of a lot more than Tokiomi as an adversary to Kiritsugu and Iri, because, well, Kirei himself fears the guy.