The Monster needs the Candidates to leave the Island with him.
I don't buy it. The Candidates, IMO, are gatekeepers just like Jacob, and until someone takes Jacob's job each of them can keep the Monster on the Island. They have to let him leave, so he needs them all in the same place in order to offer them a choice:
Let him off the Island, or die.
He'll give them all the chance to go with him, of course - he might be the Monster, but he's no monster. Only one chance, though. Too much has gone into this plan for the Monster to show mercy. He's put everything into this attempt. It seems he won't have it in him for another try.
This is why he needs his Dark Army. It's already been established he can't kill Jacob, so it's safe to assume he can't kill a Candidate, either.
I could be wrong. I don't think the Monster is evil, but I don't think he cares what happens to anyone who gets in the way of his goal.