Redmond was stunned to see how Cane took the dagger calmly. As a warrior, he knew the Alpha could avoid getting stabbed easily, but he didn't dodge it. Hell! He didn't even try. He used such an extreme way to get the dagger out of Iris' hand.
Once Cane could pry Iris' hand away from the handle, he put some distance between them, to make sure she wouldn't be able to snatch it back, only then Cane pulled out the dagger and threw it away, as more fresh blood burst out from his wound.
The way Cane took out the dagger recklessly, only aggravated the wound. Knowing what happened to the Alpha, Redmond shouldn't be surprised how well he could endure the pain, but he still was.
Redmond was stupefied, until Jace elbowed him to get his attention.