I agree that the flannel material twisted into a knot could have put those marks on Kris's neck.   I think that it also make it such that when Mac had a grip of the neck of the pj top that he was holding the pj top away from her body as she struggled, and he was stabbing the top not her body, which accounted for there being 19 cuts to the pj top but only 4 cuts to Kris's chest. 

Since those cuts to the chest were done close to death, I think that when she fought back and used her hands that he then turned her over to cut her on her back, then turned her over again to cut her in the chest shortly before she died.