Sure, she's a murderous frootloop, but there's so much about Harley that elicits sympathy... at least Ivy is there for her when she needs dusting off and putting back on her feet. This really captures that sweet friends/maybe more than friends element they have perfectly. Love it
Thing is, Ivy tries to convince Harley to stand up for herself, telling her that "Mistuh Jay" dominates her, treats her like trash, and then throws her away, at which point, she's a shoulder to cry on...
Thing is, when that happens and Harley is with Ivy, Ivy quickly becomes the dominant one herself in the partnership, even when she tries to build Harley's self-esteem.
Ivy's just as damaged as Harley in all honesty; firmly believing all humans are wanton risks to the environment to a level where she's more or less convinced herself that she's no longer human and the only solution to the problem is getting rid of people - depending on how militant she's being written at least. But, as her relationship with Harley (and a period where she looked after orphans after the earthquake in Gotham storyline) proves she can't let people go. They could conceivably "fix" each others problems, but the scars in their psyches evidently run extremely deep.
Sure, Ivy talks to Harl like she's dumb, puts her down verbally at times, and definitely is the one to try and take charge of their relationship first... she doesn't casually knock her around, though. And they've had little spats here and there but she's never seriously put any effort into trying to kill Harley. Unlike someone else we could mention. And there's got to be some irritation (jealousy?) that no matter how many times Harley reels through her door like this, and no matter how much Ivy tries to put her on the right track, Harl will just roll back downhill to Mistah J. So sure, Ivy's not a perfect partner, but she's certainly a healthier, easier-to-understand option than a casually-abusive, violent lunatic that wears his own face as a mask...
They don't even try. (One time they did try, with Amygdala, he got worse.) But the big problem is security. Go to TV Tropes and look up "Cardboard Prison" to see why Arkham is so bad. (And why Gotham is such a hellhole.)
Well, first of all, most of the staff are pretty insane themselves.
First off, lest we forget, Harley used to be a psychiatrist working there.
Then there was Dr. Hugo Strange, and Dr. Jonathan Crane (aka the Scarecrow), both former doctors at Arkham.
Lest we forget Dr. Jeremiah Arhkam himself. At first he was an okay and well-meaning shrink (but had the worst job one could have, running Arkham) only to go off the deep end himself and became the new Black Mask. (Fortunately, he seems okay in The New 52.)