As of Death of the New Gods #4, it has been shown that the killer of the New Gods was a primitive being who has been empowered by some unknown agent. This person didn't previously have power even approaching that of the New God Killer and seems to have been Anthro(maybe). The only idea I can come up with as to the identity of the benefactor of the Killer is that it's some kind of "anti-"Source or maybe a death god of Earth tradition like Neron or Satannus. I based this solely on the soul butterflies that the Killer pulls from the New Gods he kills. Souls manifesting as butterflies is found in many religions, including Egyptian and Gaelic.
However, Metron's dream and Mister Miracle's Anti-Life form, make me think it could be an aspect of an "Anti-Death Equation" or some other opposite of the Anti-Life Equation and in turn, an opposite of the Source. Since you have to pierce the Source Wall to learn the Anti-Life Equation, a Source opposite would have it's own Equation to be learned inside of it.
As far Jimmy's powers is concerned, Forager's recognition of the Source in Jimmy's eyes leads me to believe that Jimmy is being used as a conduit of the Source, as whoever created the New God Killer is blocking the Source from it's usual pathways. Jimmy's previous run-ins with the New Gods, as well as his unassuming presence, makes him the best non-New God agent to work through. The Source is maybe using him as an opposite of the New God Killer and may have Jimmy fight the Killer in the end.
More than likely these were the conclusions Stalin wanted the readers to come to, but it makes me less preoccupied with the mystery if I tell other people what I think so far.