I don’t think the argument does fail. OP is not arguing against killing in the heat of the moment in self-defence or whatever. OP is arguing specifically about executions after the fact. The only way “maximizing who survives” would be relevant is if you believe capital punishment is a deterrent, and that is arguable.
Well I think it was offensive of Cynthia to erase the original artist’s vision and it’s shameful of you to defend her.
Both of you have hurt my feelings, and therefore you are in the wrong.