Many anti-Calvinists argue against Calvinism because of what happened to Michael Servetus. Here are some resources as to how my fellow Calvinists deal with this objection.
A summary of what James White claims really happened between John Calvin and Michael Servetus.
According
to James White, Calvin risked his life to privately meet with and
discuss theology with Michael Servetus. But Servetus didn't show up.
Later on, when it was known that Calvin knew Servetus' pseudonym
(Michael de Villanueva), the city Servetus lived in, and that he was employed
by an archbishop in a major city, Calvin reluctantly gave up the
information. The man who squeezed the information out of Calvin
confessed that he had real difficulty getting the information out of
Calvin because Calvin would have rather refuted Servetus in writing than
have him executed. But because there was a governmental investigation
going on, Calvin gave in.
Servetus' arrest by the Catholic authorities was aided by Calvin having provided evidence of the real identity of Michael de Villanueva. However, Servetus eventually escaped.
Servetus then made a beeline flight to Geneva. During that time Calvin wasn't, as many
think, the autocrat of Geneva. He wasn't even a citizen of Geneva. He became a citizen six years later in 1559. Servetus entered Geneva knowing that if he was discovered that he would almost certainly be arrested for heresy. Sometime in Geneva
Servetus himself accused Calvin of heresy and attempted to convince the
Genevan authorities to arrest him on charges of heresy. Knowing that at
that time Calvin had many opponents in Geneva and that the elections
didn't go in Calvin's favor. And so Servetus tried to get Calvin's political enemies to side with him against Calvin. Servetus even filed a motion accusing
Calvin of simony. He did so in such a way that if it succeeded Calvin's
home and property might fall to Servetus.
When the Genevan authorities eventually convicted Servetus of heresy and determined he was worthy of the death penalty the ministers
(including Calvin) asked that Servetus be executed in a way that was
quicker and less painful. But the secular council refused. Even up until
the night before Servetus was executed Calvin tried to convince him of
the error of his ways in hopes that he would repent. Finally, it was
William Farel, not Calvin, who accompanied Servetus to his execution.