You guessed? You must have been pretty sure, considering you could have killed me.""I was ninety percent sure.""I see,"Clary said. There must have been something in her voice, because he turned to look at her. Her hand cracked across his face, a slap that rocked him back on his heels. He put his hands on his cheek, more in surprise than pain."What the hell was that for?""The other ten percent.

The speaker is Clary, who has just been told by someone that they were 90% sure that Clary was not the person they were looking for. Clary is slapping the person as a punishment for that last 10% chance that they could have been wrong. In other words, Clary wanted the person to know that the other 10% could have been terrifying for her and that she found the person’s words alarming. She is reminding the person that their hesitation and uncertainty could have damaged or even endangered her.