Ah... well, I can't speak for all editors, but if I had the first half of a novel and was really excited about it, I would let the author know that I'd love to see the rest when it was done.
I submitted three chapters and a synopsis to two editors I met at a very small regional SCBWI event. When I sent the 3 chaps, thinking I had a minimum of 4-6 months wait ahead of me, I was aware my MS still needed a last copyedit and a proofread. When Editor A requested the full MS, only three weeks later, complete with scribbled smiley face on her note to me, I was elated.... and horrified to find my story wasn't as ready as I thought. The second half (at least) had some serious issues which suddenly became glaringly obvious to me now that someone actually wanted to see the full. I'm sure I made a complete gaffe of the situation: I sent Editor A the first half of the MS, and explained what had happened, with an assurance I was committed to fixing the issues. Editor A quickly rejected my half MS with a form letter. No real surprises there, I guess, a sharp rap on the knuckles, and a hard lesson learned.
Unless it was about vampires, zombies, or angels. Only manuscripts ready to be sent to the printer can have those in them right now. Damned angels.
At the same time, you're right, you shouldn't be submitting a novel that isn't finished, so I'm glad you're taking this as a lesson.
I'm wondering what's the best way to handle Editor B: just to send her a letter and withdraw my MS from consideration at this point? I don't want to waste her time. But I'm not expecting that she will request the full. And I don't want to come across as Stupidity Exhibit A to someone whom I admire. How is an author, if she's silly enough to be there, expected to handle this kind of situation? Talk about arguing for one's limitations....Let her know that you feel you've noticed a couple problem areas and want to send the manuscript to her once it's really as good as you can make it. She should appreciate that. And give her an idea of how soon that might be: a month? A couple months? I don't mean to pressure you, but we like people who have a new book every year or two.