How did John Calvin reconcile killing followers of opposing religious views with the New Testament? I was reading about persecutions by Christians and was wondering if the persecutors justified their violence with passages of the bible and how they reconciled that with the teachings of Jesus.

In particular, on this website I read about Calvin: "So entirely was he in favour of persecuting measures, that he wrote a treatise in defence of them, maintaining the lawfulness of putting heretics to death."

I'm not asking about the general view of Calvin regarding capital punishment or his philosophical, civil or criminal law thoughts on it but specifically I was wondering how he reconciled supporting killing of people with other religious views with the teachings of Jesus or the New Testament. Would Jesus, according to Calvin, have supported the killing of heretics?
See this article (A Recipe for Intolerance: A Study of the Reasons Behind John Calvin's Approval of Punishment for Heresy) for one researcher's opinion.

The gist is that:

1) Calvin generally agreed that Old Testament laws were not binding on Christians, but contradicted himself by appealing to them to defend the punishment of heretics, in a way that was similar to Augustine. 

2) This combined with an ancient legal tradition that originally sprang from Scripture (the Constitutio Criminalis Carolina) but then developed on its own to form the basis of civil law. This cultural tradition permitted the execution of heretics.

3) A group he called the "Libertines" used Servetus as a test case to embarrass and discredit Calvin as a powerplay. It was a setup to sour his reputation and assume power. The defense attorney, Berthelier, was the leader of this group. The Libertines had written scathing attacks against Calvin's Institutes, making it a personal matter.