Follow me with this:
God is fed up with mankind and wants us dead.
Micheal the Archangel gets cold feet and goes rouge. OK, not bad, but here is where it could've got good. Who else would be a logical ally opposing the destruction of mankind? Satan and his legions of demons, that's who. Without souls to plunder, they are out of a job. Satan and Micheal already know each other [the devil was once an archangel himself], so they could have negotiated a temporary truce. Such an arrangement would have the audience rooting for the good guy [Lucifer] over the bad guy [God].