I called Love killing Delilah (she's basically Deb from Season 6 onwards, and Delilah was La Guerta). Fincher is Batista, dumb cop in love with La Guerta who can't quite work it all out.
As for Forty, he wasn't actually a murderer - it was Love who killed his babysitter, and he hinted he knew this. I suspect Love has killed much more people than she let on to Joe (maybe even her husband?!) and we'll find out more about that in S3!