Well I think the girl in his past was essentially a Level 3, and that didn't change her rating when she went berserk it was instead more of a "Told you so!" sort of thing with the government pointing fingers and wanting her dead.  Nastume used to be a level two, but was moved to Level three because she killed someone. Plus if she decided to rebel - with her abilities - she could cause some serious damage. All it would take is wanting /everyone/ dead and then adding her blood to some sort of distributor and... bam... hundreds maybe even thousands sick or dying. (Due to dilution, mind you.)