You're right, Harley is a very broken character. Just in the animated series they show the depths of madness that go through that relationship. There were many times where she did turn on her puddin because of something he said or did. The trial episode was a minor one, the episode where Joker was going to blow up Gotham was another, and she was almost ready to give up on him in "Mad Love" until she saw the Get Well Soon from him. I've never seen Harley as inherently evil. Insane? Oh yeah. There's no doubts about that. But I don't think she's evil. She's just overly devoted to the Joker, and about the only times she's NOT with him, she's with her other partner(in every meaning of the word), Poison Ivy. Part of it, I think, comes from the line she had when she was first starting at Arkham Asylum. "I've always had a thing for extreme personalities. You can't deny there's an element of glamour to these super criminals." The Joker played her hard. Whether there's any true feelings from the Joker is up for debate, though I think that can be attributed to his ever changing moods. Sometimes, yes, I believe the Joker DOES care, and even misses her when she's not around, often forgetting(Such as in "Harley and Ivy") that he's the reason she's gone. Sometimes he just needs his henchwoman. Basically it comes down to one sad truth. Harley's devotion to the Joker is absolute, but his devotion depends on the time of day.
Heck, this is even shown a bit in the most recent trailer for Injustice: Gods Among Us where it seems the Joker was about to stab Harley when Lex Luthor, of all people, saved her by zapping the Joker.
"Geez, we've been back together for five minutes Doc, and you're already talking about the end of the universe."
"Stupid, stupid rat creatures!"