While it goes against popular myth, the truth is John Calvin was not in political control of Geneva (ever) and at the time of Michael Servetus' arrest in that city those in political control of the city men who were opponents of John Calvin. And it was the political antagonists of Calvin who arrested Servetus (though Calvin did turn him in) and was they who conducted the trial of Servetus. These same opponents of Calvin are the ones who sentenced Servetus to death and carried out his execution.
The powers that then controlled Geneva would not allow Calvin to serve weekly Communion in Church, which he desired to do. He could not get his way on the issue of Communion in the Church he pastored because the city leaders vetoed his desires. He could not dictate what happened in worship then, so he certainly did not control what took place on the political level.
When we consider the case of Servetus we have to remember that he had already been captured, tried and sentenced to death by Roman Catholic powers. He had managed to escaped one death sentence when he arrived in Geneva. The city of Vienna, where he had been condemned, demanded his return. The powers in charge of Geneva gave Servetus the choice of returning to Vienna (and certain death) or he could be tried again in Geneva. He chose to stay in Geneva. This is probably because he knew that NO ONE had been executed for heresy in Geneva in all the years that Calvin had been there. This was not true of many places in Europe then.
Calvin did testify against Servetus Like almost everyone else in European culture at that time, Protestant and Catholic alike, he believed Servetus should be put to death. Still, Calvin pleaded with the city to show mercy and have him executed in a human way (They did not heed to Calvin's plea). Those in power checked with the other Protestant cities and the all agreed with the sentence of Servetus. In fact, Calvin was condemned by some of his contemporaries for being too lenient on Servetus.
Calvin stayed by Servetus’ side hoping to convert him to the faith and thereby see him saved spiritually and physically. He pleaded for his conversion, but it was not to be.
Remember that at that time people were being executed all over Europe for heresy (wrong as that may be) but it seems some people, in order to smeer John Calvin's name, only remember a contorted version of this one miscarriage of justice.
Calvin certainly had feet of clay, judged by today's standards there is much about him that we can condemn. But Calvin, like all of us, was a man of his times, and if we are to judge him fairly than we must see him and judge him in the context of his times. When we do that, I believe he shines brightly.