The threat has to be somewhat vague and disturbing. Suggestions of body parts in kitchen disposals, chemicals used to break down human tissue, and woodchipper incidents are a good start. Combine those with a deep, penetrating stare and a smile that suggests that there is not enough medication in the world to cure whatever is wrong with you, and he'll begin to get the right idea.
Pulling out a vegetable peeler, eyeing him thoughtfully, and asking, "If you were to skin a person, where would you start?" can be a great conversation starter.
Hope that helps.