That verse can be quite easily explained.
Satan destroys. That's what he does. He wreaks havoc and unneccessary destruction. To have heretics and atheists at war with each other brings him great enjoyment I imagine.
You see, if he were divided against himself, he would be building up in some places, and tearing down in others. That is what Christ means about Satan not being divided against himself. He NEVER builds up, but ALWAYS tears down. Whereever he goes, whatever he sees, he wants to tear down and cast into the fire.
He's doing a mighty fine job of it if he's getting his own "followers" to tear themselves apart. He wants to see the world burn, and that's what he's doing.