Having him suddenly behind her again spooked her. How had she missed it? The 'boo' part had sort of ruined the scary effect for her, and just made her more annoyed. Kate didn't know how strong this guy was, or how fast he was. But her strength and speed was greatly heightened due to her werewolf gene. This would either work, or it wouldn't. One hand flew up to slash at arm holding the knife and shove it away. With any luck she'd take a piece of his arm with her. Unless he was one of those 'can't be hurt' kind of people.
While she tried to push his hand forward, she pushed herself back, bringing herself closer to him, but away from the knife. If she managed to get his arm away, she turn her claws on his throat, having no idea that the man was as good as immortal.