No Christian, especially Bible-believers and leaders, should in any circumstances support murder in an form. He and the others who were true believers could have rejected this, as I am sure that you and I would do if faced with the same
There is a technical term for applying the morality of one time and place to all cultures. I don't remember what it is at this moment. However, your argument implies that there were NO CHRISTIANS in any church leadership throughout Europe in the entire 16th Century because executing heretics was how society functioned. That is a flawed conclusion.
From their perspective, they were responsible Elders defending those God had placed in their care from heretical teachers that threatened to lead the sheep away from God and into eternal damnation through their lies. What would YOU do to protect those that God had commanded you to protect from eternal damnation?
In occupied France, I would have been willing to join the resistance and kill Nazi's with IEDs ... if I should be called to answer to Jesus for my "non-pacifist" actions, then so be it. Let God judge my heart.
So, no, "I" would probably not have supported burning heretics ... but neither would I have been willing to stand in judgement of those that did. Let them answer to God for their hearts.