In "Vampire Knight" ep. 3, entitled 'Fangs of Repentance', Yuki seems genuinely surprised by the Level E vampires she encounters in town. It's like she had no idea vampires could be that awful.
She never seems to want to believe that vampires are bad, and even tells Zero that she probably should start listening to him and accept that there really are some bad vampires out there, only after her attack.
Why does she seem so surprised by Level E Vamps? Her earliest memory is of Kaname killing a clearly monstrous vampire. She should know and accept how bad they can be right off the bat.