Ellen's parents ignored her, and kids didn't want to hang out with her because of her sickness, so she slaughtered her parents when she was 7 years old and set her house on fire immediately afterwards.
A black cat, who was actually a demon inside a cat's body, ate her parent's souls and thanked her for the meal. He knew about her sickness, and told her that if she gave him more people to eat, he would teach her a spell to cure her sickness. So, they signed a pact and Ellen became a witch from there.
After feeding him many, many people (so many that that's why Viola's father said there was a "legend" that a witch in the forest kidnaps children) the cat gave her a body switching spell. What she wanted was to be loved, and then she met Viola, a girl who was loved.
Viola became 'friends' with Ellen out of sympathy, and one day Ellen asked her for a favor, that they could switch bodies, "just for a day." Viola foolishly agreed to so, but beforehand, Ellen gouged out her own eyes and cut off her own legs so that they could not switch back.
Viola screamed in agony in her newfound body, and continued screaming. Ellen couldn't stand to hear her own screams, so she gave her a medicine, lying to her that it would soothe her. Obviously, Viola drank it since she was in so much pain, but it was actually a throat burning medicine, so she could hardly speak.
Ellen left Viola there to rot and die, but Viola managed to use Ellen's own magic in her body to trap Ellen, making her unable to escape (beginning of the game) with the bushes and roses. So Ellen had to go back inside the house to acquire the cute little bottle, while Viola tried to recover her body back. And as you can see, Ellen manages to successfully escape with the bottle and go off with her new daddy
This info is all from the game, if you finish and replay at a certain saving point, there will be a book in the room titled "Ellen" and it tells you everything I just said, so the other info was inaccurate