Somewhere in the beginning of the movie, when we are still figuring out what Maleficent's past is, we see that she fell in love with Stefan, but was betrayed by him. He was tasked with killing her, but he couldn't make himself do it, which is why he cuts off her wings with iron (which is deadly to fairies, which Maleficent is), and leaves her to die. However, she doesn't, and then she becomes 'evil.'
