If God told you to kill your son, would you do it?

You can't say that God would never do that, because he already did do it to Abraham. Sure, you can argue that was a special case, but the precedent is there.

Alternately, if someone else told you that God had told them to kill their son, what Biblical argument could you use to change their minds? What possible appeal could you make, from the standpoint of religiously-driven morality, that would trump a person’s sincere belief that God had spoken to them?