My thoughts on pre-med vs impulse.
I'm leaning a little bit away from pre-meditation, especially if the killer was a girl. I think how I would feel if I was in the same situation, and the decision to use a knife is a very bold one. If you ever enter into any hand-to-hand combat/fight, you must be pretty confident in yourself since whatever you intend to inflict on the opponent could very well be inflicted upon yourself. If you enter into a fight with a knife in your hand, you have to have first thought of the very real possibility that the knife ends up in the target's hands and used against yourself.
Basically, you'd be aware that you're risking your own life... unless you're an experienced knife-fighter or know martial arts, or have extreme confidence in your ability to carry out the act without getting yourself killed or harmed. If it was premeditated, I'd like to think most people would have considered this.
I think if someone had planned this prior to going to his apartment, there would be a number of more desirable methods with less chance of self injury or death (poisoning or arson spring to mind immediately). Otherwise, maybe the killer was a male who is confident of his ability, or maybe there was more than one killer.
Presuming there was only one killer though, and presuming it was a lover, I think it was likely a passionate and unplanned crime committed with extreme emotion. A knife is a common object, it only takes a few moments to pass by the kitchen and sneak one into your palm. Or, perhaps a fishing knife in his room, or a Leatherman tool with a fold out blade. I actually have a knife down the side of my bed - I've always slept with a weapon of some description in arms reach, and many other guys I know do too, so it's not inconceivable that MH might have had a knife close at hand.
My mind is still open to all possibilities, just sharing a few of my thoughts...