He was offended by Zwingli's conception of the Lord's Supper and he did not approve of Zwingli's followers' propensity for violence in defense of the faith.
Zwingli believed that when the Lord said "This is my body," he meant "This represents my body." This incensed Luther, who regarded it impious. The two only met once, in 1529 at the Marburg Colloquy. They drew up 15 points of doctrine summing up the Reformation and were agreed on 14 of those -- only disagreeing on the doctrine of the Lord's Supper. At the end, Luther said to Zwingli's ally Bucer, "We are not of the same spirit."
In Table Talk, a compilation of Luther's remarks by his companions, he says, "I've bitten into many a nut, believing it to be good, only to find it wormy. Zwingli and Erasmus are nothing but wormy nuts that taste like crap in one’s mouth!" At another point he said, "I wish from my heart Zwingli could be saved, but I fear the contrary; for Christ has said that those who deny him shall be damned." Shortly after Zwingli's death in battle he said, "Zwingli drew his sword. Therefore he has received the reward that Christ spoke of, 'All who take the sword will perish by the sword'. If God has saved him, he has done so above and beyond the rule."