When was it first said in Christianity by a church leader that heretics must be put to death? Who came up with this idea and presented it?

I was wondering who and when was the first person executed on orders of a pope or if there is no name information, when was the first such execution.

So the Catholic church used their influence to sanction the execution of so-called "heretics" and now they do not. Somewhere in the middle they stopped supporting executions based on heresy. Why did they stop? Was a formal reason given? This question sheds some great light as to when they stopped and began hinting as to why but doesn't really dig deep into it. --- ***Important Note***: The premise of my question is that the Catholic church had the *de facto* ability to convict and order executions based on so-called "heresy" and that it followed through with that ability. I acknowledge that *technically* the Catholic church itself never executed anyone, but through its influence on secular government had, in effect, the ability to execute through excommunication. Some examples of this include The Massacre of Mérindol , Jan Hus , Cathar Crusade , Inquisitions , Joan of Arc , Priscillian , and the Stratford Massacre .