her own speed can hurt her which she does intentionally here

“Inertia. Saints can instantaneously break the sound barrier while moving around the battlefield. But there’s still a burden on your body. If you do not follow the rails of the optimal movements, you will destroy your own blood vessels and joints in no time. It’ll be like having your entire body slowly crushed by a suspended ceiling or a press. It looks to me like you’re trying to punish yourself. But if you’re afraid of killing, you could always just set down your sword.”

Even after all this, Kanzaki Kaori still could not kill that boy.

When he had reached the same speed and experienced the flow of time the same as her, she must have bit her lip. She must have been horrified at what a dangerous a thing he was doing. If she had simply swept his feet out from under him to trip him, he would have died so easily, but she could not bring herself to deliver a finishing blow to the boy who had wandered into an unfamiliar world.
***
But yet again, it was all in how you used it. The magicians of the ancient Rosicrucian Order had not lamented over the prejudices that befell them. Instead, they used them to preserve the secrecy of their cabal.

“Intentionally tearing apart your own body through the power of inertia, huh? I wonder if my healing magic is good enough. Then again, a Saint’s own blood probably has a healing effect.”

And she would not let it end here.

Dion Fortune crouched down and peered at the collapsed Kanzaki’s face.

- New Testament Toaru Majutsu no Index Volume 22 Reverse Chapter 4 Part 9