First off, as JBH has stated, Calvinists do not identify (or, at least, should not) identify themselves as Calvinists first and Christians second. After all, "JC" stands for Jesus Christ, not John Calvin
Secondly, Calvin was not and is not infallible. I greatly disagree with him on baptism, for example. As a Calvinist, I do not follow his teachings blindly. In fact, I do not follow him at all. As I've said many times: Calvin was essentially Augustinian and Augustine was essentially Pauline.
I will gladly disagree with Calvin when and where he is out of step with Scripture. Calvinism is not a cult, after all.
Third, the world in which Calvin lived saw no separation between church and state. In Calvin's day, to be a heretic was to be an enemy of the state. And it wasn't only in Geneva that this happened. When Luther was excommunicated by the Catholic Church, his life was in danger precisely because he was counted as a heretic by the church and, as such, he was also considered an enemy of the state--and all of Europe was that State.
We must be careful not to read 16th Century history with the eyes of a 21st Century person.
Do I understand why Servetus was put to death? Sure. Do I agree with Calvin's agreeing to put Servetus to death? No. Was Calvin the one who signed the death order? No. The major, overriding factor in Servetus being put to death was (obviously not including his heresy) the time period and state in which he lived.
The Archangel