I would rewrite a "to save the earth" route. It seems to me that the zombies only really eat people, although there are zombie dogs and some birds, those seem to be infected purposely and they seem to want to only eat humans. So there you go a crazy tree-hugger creates or steals the virus to build an army of people eating zombies that he uses to clear out people and let the land reclaim itself. The zombies of course are in a state of decay so there life span is short like maybe 10 years or something and of course this works for the crazy tree-hugger as he is looking for the destruction of all people. It is kind of a reversal of the slash and burn stuff used on the rain forest, this guy could also become a monster because he wishes to remove all humanity from him self. This is where I would start. Also on a fun note it might be interesting to build a story where people sympathize with the monster at some point. Make sure he makes some sense. Like I said this where I would start.