That is just it again, she never became 'EVIL' she became a demon. Demon's aren't evil. You have seen Merl, Angel,Clem... and others. Becoming a demon doesn't mean you become evil. That is a misconception. She reverted back to the only thing she knew how to do wouldn't you if you were all alone with no one even trying to help you. She was with these people for years, she thought they cared about her. Not even Xander went to see her... mainly because he was the one to break her heart. When Willow sees her again they act as if they are still friends, because they somewhat were. You don't have to like all your friends lol. Plus Buffy still isn't convincing me that "oh because she really wasn't completely my bff like you Willow, I'm going to wait a few months til she kills someone, so I can kill her..." Buffy as you said new something like this would eventually happen... why didn't she try to stop it, she is the slayer after all, it is her job.