You can read the 38 Articles of complaint that Calvin wrote (through his secretary de la Fontaine.

JA Wylie (a Presbyterian author writing in the 1800s who has a lot of issues, but is still really useful as a source of orthodox presbyerian 'spin') [said](http://doctrine.org/the-history-of-protestantism-2/the-history-of-protestantism-volume-second-book-fourteenth-rise-and-establishment-of-protestantism-at-geneva/#CHAPTER 21):

The Council, in coming to the conclusion that Servetus was guilty, appear to have been influenced less by his opinions on the Trinity than by his views on baptism. The frightful excesses of the Anabaptists in Germany and Switzerland, which were fresh in their memory, made the Council, doubtless, view this as the most dangerous part of his creed.

I'm not really an expert on the Munster Rebellion, but I can understand why it would scare the pants off people.

Now, the Libertines were not so much pro-Servetus as they were anti-Calvin. The trial of Servetus was just another step in a long series of conflicts between les enfants de Geneve / the Libertines, lead by Berthelier and Calvin and his supporters. The conflict was complex, but I guess at its heart you could say it was over who held the power in the city. The Council, or the Consistory? Who had the power to excommunicate especially was a heated question.

The trial was less about whether Servetus' ideas were heretical (which by any remotely orthodox interpretation they obviously were), but whether his ideas were socially dangerous (which was a far more open question).

Geneva sent messengers across the country to try and form a consensus as to what should be done, and they all came down against Servetus. At this point an officer from (Roman Catholic) Vienna arrived, requesting that Servetus be released into his custody and returned to Vienna to be executed.

Wylie again:

Servetus had already been condemned by the Popish tribunal of Vienne; the tribunal of the Swiss Reform had unanimously condemned him; the codes of Theodosius and Justinian, which still formed the basis of the criminal jurisprudence of Geneva, condemned him; and the universal opinion of Christendom, Popish and Protestant, held him to be worthy of death. To these considerations was added the horror his sentiments had inspired in all minds. Not only did his opinions outrage the fundamental doctrines of the then common creed of Christendom; they assailed with atrocious blasphemy the persons of the Trinity; and they tore up, in their last consequences, the roots of society, by striking down conscience within man, and the power of law without him. What day the Council acquitted Servetus, it pronounced the dissolution of the State, political and religious, and opened the flood-gates on Christendom of those horrible impieties and massacring crusades which had already inflicted fearful havoc in many of the provinces of Germany.

Europe, they believed, would not hold them guiltless if they let loose this plague a second time.

Servetus was sentenced to the stake. Calvin tried to have him beheaded instead, which apparently was the form of death that Servetus assumed he was going to receive. Servetus didn't find out he was to die by the stake until a few hours before the execution itself.

He was led before the court. “The staff was broken over his head,” as was the wont with criminals adjudged to death, and the sentence was then read by the presiding syndic. Scarcely had the last words, which doomed him “to be fastened to a stake, and burned alive, till his body be reduced to ashes,” fallen on his ears, when he cast himself at the feet of his judges, entreating that he might be permitted to die by the sword,” saying that if he had erred, he had erred through ignorance, and that his opinions were conformable to the Word of God.

Anyway, I guess the short of it is that Servetus had few friends anywhere. He was a wanted man across all of Europe, and his Genevan defenders did so only to score political points against Calvin. Well, that's probably a bit uncharitable. But that's my reading of it.