How so? Calvin did not believe all Old Covenant laws had been set aside by the New Covenant Jesus inaugurated. He didn't buy into the plain sense of Hebrews: “God has made the first covenant obsolete” (Hebrews 8:13). He maneuvered around Paul’s conclusion: “the Law became a tutor to lead us to Christ and now that faith has come we are no longer under a tutor” (Galatians 3:24-25; cf. Rom 10:4). Calvin dismissed this data from the New Testament and decided the moral laws in the Old Covenant laws of the Torah still applied. And killing people who perverted his pure doctrine was a moral necessity.
Calvin specifically justified capital punishment of heretics with Leviticus 24:16. “The one who blasphemes the name of the Lord should be put to death; all the congregation must stone him. Any foreigner or native who blasphemes the Name should be put to death.”
Jesus’ teaching to “love your enemies” didn’t stop Calvin from approving and promoting the death of his theological enemies. And Paul’s instructions for dealing with people who theologically disagree with you were equally ignored: “A servant of the Lord must not quarrel but must be kind to everyone, be able to teach, and be patient with difficult people. Gently instruct those who oppose the truth. Perhaps God will change those people’s hearts, and they will learn the truth” (2 Timothy 2:24-25). Calvin did not patiently discuss his differences with people who promoted competing ideas. Calvin requested beheadings, made death threats, and praised God for orchestrating the torture of heretics.
Calvin spelled out his theologically reinforced vengeance in a personal letter:
“I am persuaded that it is not without the special will of God that, apart from any verdict of the judges, the criminals have endured protracted torment at the hands of the executioner.” - Calvin's letter to Farel on 24 July (for more words directly from Calvin’s pen, read Selected Works of John Calvin)
Calvin believed God made sure criminals didn’t die quickly when tortured. This vengeful attitude and his support for outdated Old Covenant laws that legislated capital punishment for competing theologians that challenged his preferred doctrines looked more like ISIS than Jesus.