None of them seem to fit here. A mercy killer would only work if she views the world as so bad that she spares the babies the suffering in growing up inside of it, but she seems to acknowledge that she's a terrible person. This fact also makes me want to cross out the sadistic one, because I feel a sadistic sociopath wouldn't end up feeling remorse. And from what I can tell she didn't try to be the hero either, not even sure you can with some of the ways she chose to kill them. Could maybe some sort of compulsion or deeper mental issues, like hearing voices or something. There's definitely something very wrong with her.