The fact that John Calvin supported the Genevan City Council in their execution of Servetus (all according to 1000+ year old established laws in force all over Europe at the time) has nothing to do with the truth or falsehood of Servetus’ beliefs.
Of course modern Christians can logically believe that:
A) Servetus was definitely heretical (and was convicted twice by Roman Catholic courts for this—before being tried in Geneva).
B) The execution of heretics is not a good thing—and Geneva (and yes, Calvin) made a mistake. (Something many Protestants thought so at the time—even though both Reformed and Lutheran leaders all supported the execution before it happened—as Genevan leaders formally asked them).
I’ve read Servetus’ biography, in an excellent little book called “Hunted Heretic” by renowned scholar of the 16th Century, Roland Bainton. Servetus wrote his “Errors...” book early in life (as he was certainly a brilliant, if not a genius) before becoming a physician. He spent many years in hiding working as a doctor. One of his contributions consisted of discovering the link between the lungs and the circulation system—revolutionary for the time.
One personal characteristic that Bainton pretty fairly proves is that Servetus’ brilliance also manifested itself in long-term arrogance....to the point of hubris. He was known for example, in medical school, to correct and lecture his professors.
Had Servetus not tried to publicly argue, first with the the Roman Catholic Church, then publicly with John Calvin (with whom he had corresponded with in argument for years) he probably could have died in bed. So certain was he that he alone understood the errors of the Trinity—and that his genius alone could convince the most brilliant leaders of his age of his crystal clear arguments—this made him, as an individual, sign his own death warrant—as he was well aware of the (universal) religious intolerance of that age.
Again, the reality of Servetus’ heresy bears no relation to the wrongness of his execution... Nor does it make John Calvin into some horrible witch hunter. He was a man of his age—which compared to our own, had no religious tolerance, Roman Catholic, Protestant or Radical...
Foxe says...
It has long been the delight of both infidels and some professed Christians, when they wish to bring odium upon the opinions of Calvin, to refer to his agency in the death of Michael servetus. This action has been used on all occasions by those who have been unable to overthrow his opinions, as a conclusive argument against his whole system. “Calvin burnt Servetus! - Calvin burnt Servetus!” is good proof with a certain class of reasoners, that the Trinity is not true - that divine sovereignty is Antiscriptural, - and Christianity is a cheat. (John foxe, Foxe’s Book of Martyrs, P 202 )