I've gotta say, it seems to be damn near perfect. I feel depressed yet happy at the same time. The way they ended up being the last 2 things alive seems so fitting for their awkwardly passionate relationship.
Gotta say, looking back, I enjoy this ending a lot more than I would have if they had done a "I have to kill her now that she's a monster" ending.
That being said, I'm now off to read some comedy manga for several days non-stop to cheer myself up again