We know that in nature there are creatures that are scavengers.
A lady found a beautiful shell. She brought it home to place it on her mantle. It was then that she discovered that the creature that had inhabited the shell was yet in it and gave off a foul odor. She told her neighbor about her disappointment. The neighbor lady told her to place the shell near an ant hill.
She did, and soon the ants had cleaned her shell perfectly. Now, the shell could be placed on her mantle.
There is a hierarchy of creatures that do similar tasks. I submit that Satan the devil is the highest in the order. Even after the millennium, the earth will be teaming with evil people. It will be at this point that Satan leads them to their eventual end. This will be the final purification of the Earth, making it ready for New Jerusalem to descend on a New Earth, finally fit, and with all evil gone forever.
When the thousand years are over, Satan will be released from his prison and will go out to deceive the nations in the four corners of the earth-Gog and Magog-and to gather them for battle. In number they are like the sand of the seashore. They marched across the breadth of the earth and surrounded the camp of God's people, the city he loves. But fire came down from heaven and devoured them.
Revelation 20:7-9, NIV