In a most enlightening footnote, Schaff writes
The deepest ground of Luther’s aversion to Zwingli must be sought in his mysticism and veneration for what he conceived to be the unbroken faith of the Church. He strikingly expressed this in his letter to Duke Albrecht of Prussia (which might easily be turned into a powerful argument against the Reformation itself).
He went so far as to call Zwingli a non-Christian (Unchrist), and ten times worse than a papist (March, 1528, in his Great Confession on the Lord’s Supper). His personal interview with him at Marburg (October, 1529) produced no change, but rather intensified his dislike.
He saw in the heroic death of Zwingli and the defeat of the Zurichers at Cappel (1531) a righteous judgment of God, and found fault with the victorious Papists for not exterminating his heresy (Wider etliche Rottengeister, Letter to Albrecht of Prussia, April, 1532, in De Wette’s edition of L. Briefe, Vol. IV. pp. 352, 353).
And even shortly before his death, unnecessarily offended by a new publication of Zwingli’s works, he renewed the eucharistic controversy in his Short Confession on the Lord’s Supper (1544, in Welch’s edition, Vol. XX. p. 2195), in which he abused Zwingli and Oecolampadius as heretics, liars, and murderers of souls, and calls the Reformed generally ‘eingeteufelte [ἐνδιαβολισθέντες], durchteufelte, überteufelte lästerliche Herzen und Lügenmäuler.’ No wonder that even the gentle Melanchthon called this a ‘most atrocious book,’ and gave up all hope for union (letter to Bullinger, Aug. 30, 1544, in Corp. Reform. Vol. V. p. 475: ‘Atrocissimum Lutheri scriptum, in quo bellum περὶ δείπνου κυριακοῦ instaurat;’ comp. also his letter to Bucer, Aug. 28, 1544, in Corp. Reform. Vol. V. p. 474, both quoted also by Gieseler, Vol. IV. p. 412, note 38, and p. 434, note 37).*
You should always read the footnotes. Luther could be the vilest of men, offensive even to his closest friends- and not just in his attitude towards the Jews. Equally vile are all the modern haters of Zwingli, because they hate him without cause. And nothing is more vile, more wicked, and more un-christian than hating someone with whom you aren’t even really familiar.
*The Creeds of Christendom, with a History and Critical Notes: The History of Creeds (Vol. 1).