Well this is what I thought about the ending. Though that OVA only answered the question why she never killed the dude in the first episode. Anyways, in the last episode, when she was fighting the other Diclonious, she lost one of her horns. Also when she went in front of the firing squad, Lucy lost her other horn. When a person appeared at the door of Kouta's home, I think it was Lucy returning as a human, for she lost both her horns. Also, the music box which somewhat represented as her past stopped playing, and the clock that she was trying to fix started to work. So thats just my theory that the person at the door might be Lucy. Does anyone know if the manga goes past that point?