Liability, Proportionality, and the Number of Aggressors

Dr. Jeff McMahan (Rutgers University)

ABSTRACT: If a fully culpable attacker would otherwise kill an innocent victim, he is morally
liable to be killed in defense of the victim. Suppose there were a thousand such people 
appearing in rapid succession, each of whom would kill the same victim unless he is 
killed first. It seems to most people that the same justification applies in each case, so that 
it would be permissible for the victim to kill each culpable attacker in turn. Next suppose 
that an innocent person’s life is threatened by a single person who is morally responsible 
but not blameworthy for the threat he poses. He too seems liable to be killed in defense 
of his potential victim. But if there are a thousand such threateners, it seems to many 
people that it would be disproportionate to kill all thousand in defense of only a single 
victim. Why is it that the number of threateners matters to proportionality when each is 
only minimally responsible for the threat he poses but does not matter, or matters less, 
when each is fully culpable? This may seem a simple problem but it proves surprisingly 
difficult to provide a plausible explanation of the difference. I aim to show why this 
problem is so vexing theoretically and also to offer a solution.