I believe that the Devil is the evil will of man and God is the goodwill of man.
So inorder for God to detroy the devil and purify the Earth, the evil will of man would need to cease to exist.
I was watching that movie Gabriel a couple nights ago. And when the ArchAngels came to Earth they were all fucked up cuz they never experienced stuff like depression and fear. Thier whole mission was to bring light back to the Earth because since God was light, His presence couldn't come into dark places or people who had lost their faith.
Gabriel was the last one to come cuz all the other Angels had failed. But they hadn't died they just gave up. When he got to Earth he found the other angels who were still alive and they all told him that it was hopeless. They taught him tricks on how to hide that he was an ArchAngel because eventually he would die from being mortal and return back to the light because he still knew it existed.
... But his mission was to bring the Light back to the Earth and fight the fallen angels. So he started doin crazy shit like not hidin his powers to heal people and fightin the Fallen that came after him.
He started gettin pissed and was all shouting, "Am I the only one who remembers why we came here!?!"
I won't tell yall how it ends but what I got from it was in order to defeat the devil he had to heal and teach the lost and fight the ones who knew there was light but knew that God's presence couldn't abide in the darkness. And in the darkness they created they could rule however they wanted even though they knew it was wrong. They didn't want peace they just wanted to control people.
It reminded me of the greedy ass selfish people who control the corporations, churches, gangs, or any other kind of organization that bases its greatness on the oppression of others.
Just my two cents. =)