"First of all,we really do not know exactly what happened in detail between Servetus,calvin and the state. We tend to compare what seems to be an outrageous act with the way our culture is today."-Rippon
We do know that he was burned, and we do know that it was for heresy. If what we compare with today nullifies what was practiced 400 years ago, then there should be some kind of recognizable transition in history where burning heretics for heresy was always acceptable until society changed its views on it later.
However, there is no practice of this by any of the apostles, nor is it taught in the NT period. Thus there is no link that shows a consistent practice of burning heretics from the time of the foundation of the church, to the time when this became a normal practice for the RCC.
Furthermore, since when did the standards of society dictate that burning someone alive for heresy is acceptable or not? Even if it was not considered an outrageous act in our culture today, that would not make it any less evil and unbiblical.