Martin didn't slay anyone. What would be more accurate is to say that Lutheran nobility were initially opposed by Anabaptist (Radical Reformation types) peasants and lower classes.
By the time the killing started, it was a purely socio-political war of the upper class versus the lower class, with the countries neighboring the Germanic princedoms also chipping in.
However, Luther's pamphlets against the Church and the hierarchy, often worded with "liberal" phraseology, now led many peasants to believe he would support an attack on the upper classes in general
But his letters were contradictory and though Luther sympathised with some of their grievances, (shown in his response to the Twelve Articles of the Black Forest in May 1525), but he reminded the aggrieved to obey the temporal authorities
During a tour of Thuringia, he became enraged at the widespread burning of convents, monasteries, bishops’ palaces, and libraries. In Against the Murderous, Thieving Hordes of Peasants, written on his return to Wittenberg, he explained the Gospel teaching on wealth, condemned the violence as the devil's work,
and called for the nobles to put down the rebels like mad dogs:Therefore let everyone who can, smite, slay, and stab, secretly or openly, remembering that nothing can be more poisonous, hurtful, or devilish than a rebel ... For baptism does not make men free in body and property, but in soul; and the gospel does not make goods common, except in the case of those who, of their own free will, do what the apostles and disciples did in Acts 4 [:32–37]. They did not demand, as do our insane peasants in their raging, that the goods of others — of Pilate and Herod — should be common, but only their own goods. Our peasants, however, want to make the goods of other men common, and keep their own for themselves. Fine Christians they are! I think there is not a devil left in hell; they have all gone into the peasants. Their raving has gone beyond all measure
Lutheran was NOT speaking as a religious leader but as a conservative.
Lutheran clergy were not involved in the massacres but Anabaptist "elders" definitely were involved in the wars.
Luther's confusing statements can be viewed as one of the instigating forces behind the rebels but not as the primary force. Luther's statements against them (I hate to admit it, but I agree with his outrage, if not with his over-the-top (but justified) reaction) emboldened the nobility.