"Why execute someone if you can use their lifeforce to store for magic?"

So yes, exactly. A different type of execution. It is mercenary and vicious, but it is simply a part of life. Istredd thinks about it as much as Yennenfer does, very little, because that's the sort of thing you have to get used to. Their sphere is fairly cruel and immoral. He is empathic and cares about people more than most mages, but even then after so many years, you see enough death and mayhem to move on.

"I don't know. They did throw all the Summoned into the dungeon when they first arrived here last year, so it's possible. I haven't heard of experimentations."

But it could very well have been done. Istredd was told this by Yennefer and Geralt, he doesn't know if it's common knowledge in the castle itself.

"I truly don't know what they want from us. To get to the Singularity, I assume, the only thing they can't do themselves, but that we can."