The millstone around the neck thing that Jesus talked about was for his unforgivable sin, so far as I am aware. And that sin is to de-convert a Christian.
Mat 18:1 At that hour the disciples came near to Jesus, saying, "Who, now, is greater in the kingdom of the heavens?"
Mat 18:2 And Jesus having called near a child, set him in the midst of them,
Mat 18:3 and said, "Truly I say to you, if you may not be turned and become as the children, you may not enter into the kingdom of the heavens;
Mat 18:4 whoever then may humble himself as this child, he is the greater in the kingdom of the heavens.
Mat 18:5 And he who may receive one such child in My Name, receives Me,
Mat 18:6 and whoever may cause to stumble one of those little ones who are believing in Me, it is better for him that a weighty millstone may be hanged on his neck, and he may be sunk in the depth of the sea. (LSV)
Jesus was not talking about actual children.
I am atheist. OP is Christian. What Jesus is saying there is that if I were to turn you away from believing in Jesus, then it would be ME who should be drowned.
So yes, Jesus was pro death sentence. But this is the only case where he said it is a requirement.
So if you are a Christian, you still have to forgive.