Elena was surprisingly agreeable, not that she had been argumentative with Katherine lately or anything, but in the same position, Katherine wasn't sure she would have willingly taken Rose's blood. she hadn't really considered the benefit of killing herself and returning as a vampire until the blood was already in her system. Though she had figured Elena would ask Katherine to help her to her own cabin and leave her there. Maybe Elena actually realized that Katherine was determined not to leave her alone.
"I wouldn't let you invite me in in this condition," Katherine replied as Elena tried to get up. She watched the girl, still weak, fall back to the ground. She stood shifted positions so that it would be easy to pull Elena up with her and reached to grab her shoulders and pull her up slowly as she started to rise.
Once they were standing, Katherine allowed Elena to lean against her. "You're welcome," she answered, not really wanting to talk until they got back to the cabin. She would ask Elena who had done this to her then--it was obviously not an accident--and she'd work it out with the Salvatores who got revenge.