The Albigensian and Waldensians were simple Christians. There was no justification for killing them. It was a big fat worldly mistake.I pay no compliment to those whom celebrate (or would even repeat?) those type of mistakes.
The Lord's kingdom is not of this world. Missing that, is to miss a crucial truth.
Vengeance is the Lord's. It does not belong to men, though I must say there is much competition nowadays amongst those whom would take it upon themselves to "do God a favor" by killing others, in his name.
There is no shortage of those whom run to do so. They all have one thing in common. They congratulate themselves for doing it! That particular sickness, is most assuredly not of God.
Neither simple nor Christian. They were leading true Christians astray. St. Paul was absolutely correct.