First thing I notice in this case is that they don't release what type of object was used for the stabbing. That tells me unusual, as in probably not a standard kitchen knife. Unless Carmen collected odd and unusual items (I have a sword collection on my wall, a few battle axes, a morningstar or two, for example), that means the item would have to either have been brought with the assailant, or it was something that everyone might have and no one could tell if it was missing (screwdriver, meat thermometer, letter opener, pencil, scissors, etc).
Since my impression of the words "Get off me" indicate that it was unexpected, and that she expected her words to be honored, I have the impression of someone who was a good acquaintance, at the very least, who came hoping for a romantic interlude (you know what I mean). The scenario I see is perhaps them sitting on the sofa or edge of the bed, the guy makes his move, and she screams "Get off me!". At that point, rebuffed, he puts his hand over her mouth and grabs whatever the item is to stab her. Ergo, he did not have to search for it, so it must have been within reach or on him.
What could have been within reach if she were in a position where she had to yell "Get off me!"? If it were not as sharp as a knife, it might have taken several stabs before she stopped struggling. Then, angry because everything was now "spoiled", he just kept going. (Ever seen a guy who is really angry punch a wall several times, even though his hand is now broken? Not everyone has good anger management skills.)
She yelled "Get off me!" not "Don't hurt me." At that point, she didn't think her life was in danger, and most likely did not see a weapon. What kind of weapon could someone carry that another person would not notice? A pocket knife, a switchblade if the weapon were like that, then I'd say he brought it with him, otherwise he grabbed what was handy. If he grabbed what was handy, he did not intend on killing her, he just screwed up. That, to me, says somebody moving away, disappearing, committing suicide, or even getting married suddenly for no apparent reason right after.
That's my first thought.