I know that there's really no evidence for this, but Lady Rainicorn gives me the impression that she just would never eat humans.
She SEEMS like a normie at the surface level, but
when you get to really know her, she's really a cool individual and independant-minded person. My point is, of anyone, she seems like the type of person who would solitarily recognize the fact that killing and eating sapient people is pretty messed up. And she'd refuse to eat LITERALLY PEOPLE. That and humns are rare and she's not selfish enough to make them go extinct just for a snack.
I don' think Lady has ever really wanted to eat Finn. Alternativey, she DID find Finn before Jake, and WAS going to eat him, but then fell in love with Jake. Or Bubblegum told her not to eat him, because he protects the Candy Kindom or w/e.