It's not about what Calvin says, but what the Bible says. Whether Calvin murdered or not has no bearing on what the Bible says. It should cause us to question Calvin's teachings, according to 1 John, but it doesn't mean that we completely rule out everything he said either. E.G. If a false teacher (I'm not making an accusation, just an example) teaches that Jesus Christ was God incarnate, but then goes kidnaps, rapes, and murders someone, then is prideful of what they've done, we do not say, "Well, then Jesus wasn't God incarnate because so-and-so taught it and was a hypocrite."
I believe it was Rich Mullins who said: "God spoke to Balam through his ass, and God has been speaking through asses ever since."
Should it cause us to question what we believe? Yes. Should it cause us to be concerned about taking everything Calvin says as the Word of God? Yes. Should we then deny everything Calvin taught? No.