Even if the atheist was a moral nihilist (of course he is conflating atheism and nihilism), it still would not be rational to carry out the action because we would hope that society's condemnation from people with moral systems and appropriate deterrents (e.g the risk of getting caught and getting a life prison sentence) so even saying that moral nihilism will lead to mass murder is wrong, so long as a sufficiently large percentage of the population believe in consistent and sensible moral systems.

That's an argument against promoting moral nihilism.