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I still think it's Amy as she's the least likely suspect. It would be too obvious if it were Zak, but it could well be Moira. For me it's between Amy and Moira.
I think it's Moira. She has lost her family because of Cain. He did chase her till she gave in. Stupid woman. Then he made sure John got to find out. So she did have a big reason to do it.