I play Minecraft with my 9 yr old nephew and he's a mostly
  gentle soul with a really wicked cruel streak I try to point out
  to him.

  One day, we had some NPCs - they had names on their heads and
  their professions. Not feeling in a peaceful mood, he rounded
  them up to drop them in lava via a trap door in a cage.

  So I walked around the cage and said, "Chris the Baker, you must
  pay for your crimes...." and the like.

  In the chat, nephew writes, "Oh, they didn't do anything wrong."

  All I had to type back is: "Oh. "

  Well, within a few minutes, he had modified the roleplay so that
  he was the evil Wizard and I was to break them out when he had
  his back turned.

  *Much* better scenerio. I do what I can. I suspect he'll always
  have a slight "revenge" streak in him, but if he's at least
  aware of social implications [or just pretends around me], I'm
  ok with that.