Film it. Bring in a cute bunny and tell the perp its pregnant and healthy.
The bunny is suspended over a blender and the defendant is told if he pushes the big red button a small motor will lower the bunny into the blender for a painful death of the bunny and her progeny. Otherwise the bunny will be safe.
If he presses the button, execute him, he has no conscience, no humanity.
If he doesn't press the button, ask him why. He'll say "Because that would be wrong". So he knows the difference between right and wrong. Execute him, he knows what he did was wrong.
Put the bunny back in her crate. BTW the button does nothing except light up for the film. Keep the procedure secret of course.
Next?
My bet? This guy presses the button:
Merry Christmas ;-)