In light of the day he was living in, his attitude was comparatively compassionate. By today's standards it is harsh and cruel.
But don't forget that the Protestants had a lot of barbarism and atrocity in dealing with heretics to unlearn.
It would have been nice if it happened overnight, but it didn't.
It's ironic how Calvin is so condemned for his attitude towards heretics by members of an organization whose history of dealing with heretics is less than exemplary.
Oh well. Do you expect any different?