...Caster was able to hold multiple Servants. She had broken my contract with Shirou, and instead took me as her own Servant.

[ She is REALLY angry about that, as it shows clearly on Saber's face. She is usually so much calmer, but thinking about Caster makes her upset. ]

That is wrong...Assassin was Sasaki Kojiro. I defeated him on the steps of the Temple myself.

[ Then she pauses, and decides to be honest with Sakura. ] Shinji had become the vessel for the Grail. It had become corrupted by some force. I had to destroy it.