Who was the first person on record who was killed by a Christian or group of Christians for religious disagreements?

I am trying to learn when did things start to go wrong in Christianity and when did it start to change from a peaceful, loving religious group into a movement that engaged in war and physical repression, including executions, in the Middle Ages. Therefore I was wondering who was the first person killed by a Christian, two Christians or a group of Christians due to religious disagreements.