Why save
If the Matou family were ordinary people and had nothing to do with mystery, Caster wouldn't find it strange.
But the Matou family is not an ordinary family. Not only that, but they are also participants in the Holy Grail War and have summoned Servants. This is very strange.
The theme of the Holy Grail War is fighting. The Master is fine, there are many ways to protect himself, such as severing the contract and using the Command Spell of Light, or directly seeking shelter from the Church, but the only fate for the Servants who are not on his side is death.
Although there is no rule in the Holy Grail War that a Master can only contract with one Servant, under normal circumstances, a Master will not contract with two Servants at the same time. There are two reasons for this:
First, the supply of mana. Maintaining the existence of servants requires a large amount of mana. An ordinary magician has to do his best to supply a servant (except for those who have their own mana recovery like Altria). Even a first-class magician can hardly support the mana consumption of two servants unless he uses extraordinary means such as sacrificing lives or devouring souls.
Second, the final wish. At the end of the Holy Grail War, only one pair of wishes can be fulfilled, one master and two servants. What should be chosen in the end
In Caster's opinion, the most convenient and normal approach is to kill him with one blow while he is weak.
Shinji was of course aware of this principle, so he turned to Sakura who was clearing the cups and plates and asked.
"Sakura, why did you save her?"
"When I saw her lying on the ground alone, I couldn't help but..." Ying lowered her head involuntarily and answered in a low voice.
"I can't help it... You—" Caster's eyes flashed with a strangeness, which quickly turned into anger, "—are you feeling sorry for me?"
"I..." Ying's shoulders shrank, not knowing how to answer for a moment.
"Yes, we just feel sorry for you."
She didn't say it, but someone else said it for her. It was her servant Medusa.
"Rider, stop, stop talking."
"No, I'm going to say it."
Sakura tried to stop her, but Medusa wouldn't listen.
Although they have not been together for a long time, the relationship between Medusa and Sakura has become very good. Medusa protects Sakura everywhere, and they are indeed a golden pair summoned by compatibility.
"You lost your Master, and you're injured. Your magic power is exhausted, and it's difficult for you to walk. You make those inappropriate noises while walking. Isn't this pitiful?"
"You... you all heard that?"
Caster obviously recalled what she had said, and her long ears twitched violently. This was the only difference between her and modern people, her ears were very long, like an elf in a fantasy novel.
"Yes, I heard it. I heard it very clearly, every word."
"Hmm!" Caster's momentum dropped instantly.
"Everyone calls me a witch, and this is what happened. I clearly—"
"That's enough. Stop talking."
This time it was Caster's turn to panic. It was one thing to reveal one's true feelings before dying, but it was another to have someone repeat them to one's face after one had recovered.
"—I just want to go back home."
Medusa still refused to give in.
"That's enough! I told you to stop talking!" Caster was irritated and said angrily, "You obviously don't know anything..."
"How would we know if you don't tell us?"
"If I tell you that I killed my own Master with my own hands and that I got hurt in order to terminate the contract between me and my Master, would you still feel pity for me?"
"... "
Caster's roar silenced Medusa.
Sakura, Fujino and Kirie all looked surprised. According to Yustisa's monitoring, Atram had only summoned his servant for less than a day. How could he have killed his Master in such a short time
Only Shinji, who knew Caster's background, remained calm: "There must be a reason for killing people, and you don't look like a lunatic who kills people for fun."
"Hehe, you're wrong. I've always been called a lunatic and a treacherous witch. I don't need a reason to betray." Caster's voice was sharp, and the quiet and pure impression he had given people before was gone.
"Witch of betrayal... I see, you are the princess of Colchis."
Combining the knowledge granted by the Holy Grail and certain characteristics of Caster, Medusa determined her identity. Colchis is a country in Greek mythology. The famous Argonauts found the Golden Fleece here, and the princess of Colchis is Jason's wife Medea, a witch who betrayed her family and then her husband.
"Do you regret saving me?" Medea asked coldly.
Before Medusa could answer, laughter rang out in the room, Shinji's laughter.
"What are you laughing at?" Medea clutched the sheets tightly with both hands:
"I am smiling because of your Master, Atram Kaliashta."
"Do you know him?"
"I know him, but not very well. Sakura told me that his workshop caught fire. Was it you who did it?"
"Yes, it was me. I was the one who burned him to death in the end."
"When he was in England, that guy compared one of his magic tricks to the fire you used to burn your love rivals. Then he himself died in such a fire. Do you think it's funny?"
However, except for Shinji, no one else in the room laughed. Medea was not in the mood, and the others could not laugh. Shinji touched his nose and said in a somewhat uninterested tone.
"If your Master was someone else, I might have regretted it. Atram can only say that he deserved it. That guy has a bad mentality. He is arrogant but also very inferior. Specifically, he can't stand others being stronger than him. When he saw a God-era magician like you, his mentality collapsed. I guess he did a lot of excessive things - maybe it's also because he was born in the Middle East and looks down on women. Although you said that betrayal doesn't need a reason, you have a wish that you want to fulfill, and you know the consequences of losing your Master. I don't think your so-called "rebellious nature" is more important than your wish."
"... "Medea was speechless.
"And those women and children. If you were really crazy, why did you let them go? Are they related to you? Impossible, then the answer is obvious, you are not as bad as you say, and we will not regret saving a woman who has saved so many lives - do you like this answer?"
Shinji looked at Medea with a smile, but Medea lowered her head and looked at her hands, not looking at Shinji.
"You, what are you going to do with me?"
"Letting you go is impossible. The reason you can stay in this world is because of the effect of the workshop. If you leave here, you will disappear soon because you lose your possession. Even if you find the possession and the source of magic power in time, I don't want to have another opponent for the Holy Grail. So, I have a proposal - come and lend us a hand."
PS: The mana consumption issue is shown very clearly in the UBW animation. When Rin drags down Acheron, Shirou's inherent barrier has reached its limit. Shirou himself has twenty-seven magic circuits to supply mana, so you can see how huge the consumption is.