Jesus Christ never slandered. He spoke truth, and those who bore the brunt of His truth felt slandered. By the same token, we do not need to slander the “Prophet” of Islam; we only need to speak the truth of the Gospel and point out the lies of Islam. Those who follow the “Prophet” will feel slandered, but that is their problem; it either leads them to repentance or to damnation, and to which it leads becomes each Muslim’s choice.
In the sense of “4. malign, vilify, revile.” He exposed the false teachers, false prophets, and money changers using whips and strong words. So did the Apostle Paul, Jude, Peter, and others.