Because either way he wins. If the archmages kill the deegans crew, then that threat is removed. If the deegans kill the archmages, thats one less group of people with the power to oppose him. If some deegans and mages both die, then both sides are weakened, and may be unwilling to work together. Even if the absolute worst had occured, and the deegans through the magical might of marysueishness had freed the mages from mind control, he still wouldnt have been any worse off. So dj could not lose with this plan.