My guess is, god gets bored.
Armageddon's over, the earth has been cleansed, only his sycophants have survived. All is perfect. After a millenium or three god gets bored. He sets up some damn loyalty test. Of course, not everyone passes the test. So, those who don't pass the test die. BUT, god has to do something about that so he looks good. Jesus says, "I did it last time. Someone else can go down to earth and die for these bozos." So some other angel does it, suddenly jesus isn't numero uno anymore. He gets pissed, throws a little revolt, becomes the new satan. Satan is saying, "I told you so but NOOOOO, you wouldn't listen to me. This is bullshit, but god gets away with it because he's all-powerful. But don't worry, he just takes it out on the bozos on earth. He isn't going to do anything to us."