Thank you for the link. I've copied part and paste it here:

Calvin had Servetus arrested when he came to Geneva, and appeared as his accuser. He wanted him to be condemned to death, but not to death by burning. On August 20, 1553, Calvin wrote to Farel: "I hope that Servetus will be condemned to death, but I desire that he should be spared the cruelty of the punishment" -- he means that of fire. Farel replied to him on September 8th: "I do not greatly approve that tenderness of heart," and he goes on to warn him to be careful that "in wishing that the cruelty of the punishment of Servetus be mitigated, thou art acting as a friend towards a man who is thy greatest enemy. But I pray thee to conduct thyself in such a manner that, in future, no one will have the boldness to publish such doctrines, and to give trouble with impunity for so long a time as this man has done."
Calvin did not, on this account, modify his own opinion, but he could not make it prevail. On October 26th he wrote again to Farel: "Tomorrow Servetus will be led out to execution. We have done our best to change the kind of death, but in vain. I shall tell thee when we meet why we had no success." (Opera, XIV, pp. 590, 613-657).