You could always say, " at least Jehovah will have less people to kill during Armageddon. It will usher in the new system faster. How does that make you feel?"
Jehovah's witnesses are emotionally driven people. Therefore, you must reach their emotions, because they aren't really into thinking. That's why the society publishes so many illustrations in their books and magazines. To Keep them child like. If you create an illustration while providing your counter argument it would be useful.