Great thread, and once again,coming in late <Just call me the White Rabbit>. I had just had most of a post written when the internet gremlins decided to close explorer,so here's my attempt at reconstructing it....
(SPOILERS! --posted by admin)
At one time I powerwatched these scenes repeatedly for stuff I was writing,as I needed to think out the POV of all the participants. My conclusion on Quentin was that the attack was a surprise and he didn't ACT so much as REact. I think his INITIAL reaction was just to stop her, and things got out of hand. One thing that only a few posters took into account was his previous stabbing by Jenny. We are shown that he has no recollection of his time as a zombie -- but his memories of the stabbing by Jenny are as clear as the proverbial bell, and he has this understandable reluctance to repeat the experience. I would compare this to the instances of cops who are shot in the line of duty -- then overreact BIGTIME the next time a similar circumstance occurs. PTSD wasn't known then (as most mental conditions weren't) but here's someone who was attacked and nearly killed,being attacked again by the same person under similar circumstances. This is known for causing the victim to relive the circumstances of the initial attack. Only this time,not as a helpless victim knifed before he could defend himself, but in circumstances where he has the upper hand. THis to me is the most plausible explanation, as it covers Quentin's continuing, the expression of rage (and to me it looks like he is looking at her, but doesn't SEE her,if you know what I mean) -- and also what no one has mentioned,his reaction right afterward. Once Jenny is dead, Beth's cries FINALLY get through to him and he looks down and actually FOCUSSES on what he has done.His initial expression seems to me a cross between horror and about to vomit -- then he runs as if the hounds of hell are on his heels. Remember too that the early Quentin tended to talk big and make a lot of threats that he never followed through on-- and there's a lot of difference between planning murder in the abstract,and actually killing someone with your bare hands.
Also, as MagnusTrask points out, its not important in the long run since she is still dead. I would qualify that, though to state that it matters as regards where and how the character develops from that point on; his experiences and perceptions of the experience influencing the way he changed after that.
As to the remorse, Gothick points out the one scene right after he is cursed. That seems to be to be simple human nature. Once Jenny was dead, he didn't see that Jenny anymore, but would be remembering the early one, the one he fell in love with. There's also a later scene, when Jenny comes to him in a dream,asking him not to bring Lenore to Collinwood. He's talking to her, and telling her how he wishes things could have been different for them. I can't see that as anything but genuine remorse, since there's nothing he wants or can get from her. He doesn't even ask her to forgive him. Perhaps thats because, as with Beth's ghost later on, he couldn't ask for or accept her forgiveness because he wasn't able to forgive himself.
Jeannie