In the most notorious case, when he was a powerful leader in Switzerland, he had Michael Severtus arrested, tried on charges including blashphemy and insultling him (Calvin).
Severtus was convicted and was burned alive at the stake with the last remaining copy of his book, Christianismi Restitutio, chained to his legs.
Such a moral monster as Calvin can not instruct me in matters of faith. He is of a peace with the Puritans who also conducted witch trials and burnings, and the leaders of the Inquisition. These are among the darkest parts of Christian history. Those who had leading roles in these atrocities are not worthy of devotion today.
To understand better what happened between Calvin and Servetus, do a thought experiment with me. What if, instead of pushing anti-Trinitarian, anti-Gospel doctrine, Servetus was a serial child rapist? What if Calvin had warned him in a letter that if he ever came to Geneva, Calvin would make sure he didn’t get out alive (and Calvin apparently did so warn Servetus)? What if the Genevan council had decided, after consulting with several other Christian jurisdictions, that cruel and painful capital punishment was appropriate for such fiends? And what if Calvin had pled instead for a merciful execution, and failing after intense effort to persuade the authorities (over which he had no real control), he spent time comforting and praying with Servetus (as Calvin did)? He would almost appear to his peers as too soft on such heinous crime, would he not?
And yet, in that time, there was no distinction between crimes against the state and crimes against the church. It does not appear so to our modern eyes, but a crime against the body was actually a lesser offense than a crime against innocent souls. Therefore no crime was greater than heresy, and the gut-wrenching revulsion we feel toward the child rapist would be what they felt, in their time, for a purveyor of false doctrine.
As for your further assessment of Calvin’s character, please remember that much of Calvin’s history was written by his mortal enemies. Picture some future generation where the only record of Sarah Palin was whatever leftist bloggers had written about her, and you might get an idea why your list of broad derogations against Calvin is suspect, at best.
The bottom line however is this. It is an accident of history that Calvin’s name has become so strongly associated with the Biblical doctrines concerning God’s sovereign grace in salvation. If the imperfections of Calvin bother you, ignore Calvin. But you cannot ignore Paul, and you cannot ignore Jesus, when they teach the identical truths. For me, I take what I can from whomever God blesses with insight, be they a saint and pillar of the church, or a scurrilous knave like Balaam. They will stand before God and give account for their errors, and I will give account for mine, only I hope that my errors do not include casting away God’s truth because some have held that truth imperfectly.