I need quotations from church fathers who were against killing heretics…
First of all St. John Chrysostom (347-407), in his Homily 46 on Matthew, which deals with kingdom of heaven compared like a man who sowed good seed in his field. Bu while all were sleeping, the enemy came and sowed weeds among the wheat, and went away. When the wheat sprouted and formed heads, then the weeds also appeared. The weeds are heretics and are not to be pulled out, lest the faithful be be pulled out at the same time. The Church is to learn how to silence the heretics...
What then does the Master? He forbids them, saying, Lest haply ye root up the wheat with them. And this He said, to hinder wars from arising, and blood and slaughter. For it is not right to put a heretic to death, since an implacable war would be brought into the world. By these two reasons then He restrains them; one, that the wheat be not hurt; another, that punishment will surely overtake them, if incurably diseased. Wherefore, if you would have them punished, yet without harm to the wheat, I bid you wait for the proper season.
But what means, Lest ye root up the wheat with them? Either He means this, If you are to take up arms, and to kill the heretics, many of the saints also must needs be overthrown with them; or that of the very tares it is likely that many may change and become wheat. If therefore ye root them up beforehand, you injure that which is to become wheat, slaying some, in whom there is yet room for change and improvement. He does not therefore forbid our checking heretics, and stopping their mouths, and taking away their freedom of speech, and breaking up their assemblies and confederacies, but our killing and slaying them. - Homily 46 on Matthew
Before Christianity’s triumph over paganism, its major tool in punishing heretics was excommunication. From the end of the fourth century the emperors generally felt bound to use their power to preserve orthodox doctrine. Penalties for heretics included confiscation of property, banishment, and death.
In 385, Priscillian, a bishop in Spain, was the first person to be executed for heresy, though this sentence was roundly condemned by prominent church leaders like St. Ambrose. Priscillian was also accused of gross sexual immorality and acceptance of magic, but some sort of politics may have been involved in his sentencing.
It seems most of the Church Fathers were against the execution of heretics, but were in favour of types of sanctions such as excommunication, exile, etc.
St. Ignatius, gives us a modern-sounding warning: “Where God builds a church, the devil builds a chapel close by.”