Early on, Kate tried to save the colonel but left him to die horribly when the situation became hopeless. Later on she risks her own life to save Sasha from an equally hopeless situation. Motherly instincts coupled with lingering guilt, right? Wrong. She knew that Sasha was Alex Trusk and she needed to get her out alive.
It may seem kind of strange that Knife Nut Simon keeps getting his ass handed to him by Kate at first. (Noticeable in the many scenes where he tries to manhandle her or restrain her. Excluding the first instance where he catches her off guard, he has been shown to be completely incapable of effectively restraining her, such as in the room with the spinning cube, where she punched him in the face when he tried to restrain her, or when Simon snapped and tried to threaten her with a knife, which she effortlessly redirected to stab him in the eye, or finally, when she managed to overpower Simon in the final fight and got him to stab himself, after which she one-hit ko'd him with a karate chop. Although dismissable at first due to the observation that Simon had a thing for her (at least in the earlier half of the movie), the ending reveals that Kate was actually a professional hired by Izon, whose mission was to steal the necklace back from Alex. This also explains why she seemed so remarkably calm when Simon "held her hostage" in the beginning (which even he comments on about how she manages to retain her sense of humor even in this situation) - she wasn't afraid of him at all.