Father Kinley's prophecy said that "when the devil walks the earth and finds his true love, evil will be released into the world." Since we learned Eve isn't Lucifer's true love and Chloe seemingly is, it isn't a wild guess to assume Michael might be that evil. He's a really bad guy, though I guess it might be much to call him evil. If Michael isn't the evil prophesied, he might be the one who brings evil into the world. It's safe to say that he's willing to do anything and everything to ruin his brother's life. If evil breaks out, Lucifer will have to go back to his station in hell to sort it out and Chloe definitely won't be following him down there.