Calvin's involvement and even leadership in having a man guilty of heresy put to death according to the civil laws of his day does not seem something that can be denied based on his personal letters.
Here is the one most often quoted as the strongest recorded statement from Calvin on the Servetus affair is a 1561 letter from Calvin to the Marquis Paet, high chamberlain to the King of Navarre, in which he says:
Honour, glory, and riches shall be the reward of your pains; but above all, do not fail to rid the country of those scoundrels, who stir up the people to revolt against us. Such monsters should be exterminated, as I have exterminated Michael Servetus the Spaniard.
This shows he openly admitted his involvement and direct leadership in the affair.
My opinion on Calvin's sinfulness
I think the question is not whether Calvin sinned or not, for he should have sought to remove the ability of the sate to punish heretics, but rather how big was his sin.
It is difficult for us to imagine a world where heretics were considered just like thieves and murderers liable to capital punishment as under the Old Testament. This does not excuse Calvin at all in my mind, but when we cast the first stone we are probably just as guilty ourselves in the unwarranted anger we have had against our neighbors.
But I tell you that anyone who is angry with a brother or sister will be subject to judgment. Again, anyone who says to a brother or sister, 'Raca,' is answerable to the court. And anyone who says, 'You fool!' will be in danger of the fire of hell. (NIV Matthew 5:22)
The sins of the world at that time appeared to have infected Calvin. I do not think we can defend him, or overly judge him. After all Calvin was just a man, and though pretty clever, I have never imagined him the most holy. I find him sometimes precocious and annoying on a personal level. Even though I agree with much of what he wrote, I often feel as though I would not have liked him personally. Luther I feel I would have loved.
I say this a a long-term Calvinist who is not really surprised or bothered in finding out Calvin's guilt in this matter. We sometimes romanticize church leaders above the realm of sin, and clearly such views of men are ungodly. Furthermore at a time in history so different from our own, we need not fall into shock over their failings. They would probably faint if they had a glimpse into the great sins of our generation.
I suppose some might not like my indifferent attitude but it comes from a man that believes in the total depravity of human nature, even more than Calvin. The sin of anyone does not surprise someone as skeptical as me, neither do I believe those who claim to be so much holier than Calvin and ready to 'cast the first stone.'
Let's keep in mind the greatest sin is rejecting the love of God in the gospel, either by our doctrine or by our actions. Servetus probably did it both ways, every day, but Calvin also failed terribly in some of his actions also, especially in this one.
Does it help to recall that King David was more guilty in his sin, as he first slept with the man's wife before 'arranging' his murder?