Steve Griffin, over at his blog here, does a good job of stating what actually happened as well as dealing with some other objections. Here is a snippet:
One of the most common and unfair charges leveled against John Calvin is, “Calvin burned Michael Servetus at the stake!” This, of course, is patently untrue. Servetus was indeed burned at the stake, in Geneva, for heresy. But Calvin is not to be impugned. This was a civil trial carried out by the State, not the Church. Calvin was not the judge. He was not a juror. He was not the executioner. What he was, was the prosecution’s star witness. And why wouldn’t he be? Who would be a better witness in a trial regarding theology than John Calvin?You can find the full articles here and here. I highly recommend the blog. Read and enjoy.
Calvin, via private letter, warned Servetus [who was wanted in several European States] to not come to Geneva. This letter was to no avail. Servetus brought his particular brand of heresy [he denied the Trinity] to Geneva and was subsequently arrested by the magistrate, tried for heresy, and condemned to death by burning. Calvin pleaded with the authorities for a more humane execution. This also, was to no avail.
Soli Deo Gloria!
For His Glory,
Fernando