Maybe since it was Buffy's worst nightmare, she had to have her good conscious alive and kicking to realize that she's turned into a vampire. I mean, it wouldn't really be a nightmare for her if she was evil with an evil conscious because what would she have to fear then?
I don't think it meant she was evil when she was stating that she was starting to get hungry. She wanted to hurry because she didn't want to have to think about drinking blood.