On the 11th and 12th of October in 1526 Zwingli took the stand in the trial of one Jakob Grebel (yes, the father of THAT Grebel named Conrad, the syphilitic anabaptist) and three others, who were in the dock for treason.
J. Grebel was a patrician of the city and a leading supporter of the Church of Rome and as such he had been blessed with certain monetary inducements. The city council saw such things as influence peddling and so viewed them as treasonous. So, in 1526, said Grebel was on trial for inappropriate receipt of foreign funds and monetary gifts (evidently he was something of a greedy chap). After being tortured he admitted his crime of treason and was executed on 30 October of that same year.
Though, technically speaking, Zwingli was simply one witness among many whom the Council heard, his testimony was tremendously important. As G.R. Potter observes, had Zwingli made some sort of plea for the old man’s life he surely would have been heeded, his authority in the City at that point so well established that the Council would have granted such a request. But he didn’t. Instead, he allowed a powerful foe (and make no mistake, Grebel was a powerful foe) to face his fate alone.
It’s a black mark on Zwingli’s career, to be sure. Again, as Potter suggests, Zwingli at this juncture behaved more like a politician consolidating power than a Pastor in caring for souls.