==> probability/killers.and.pacifists.s <==
Regardless of whether you are a pacifist or a killer, you may disregard
all events in which a pacifist other than yourself is involved and
consider only events in which you are killed or a pair of killers other
than yourself is killed.

Thus we may assume that there are N killers and yourself.  If N is odd,
your odds of surviving are zero.  If N is even, it doesn't matter to
you whether you are a killer or not.  So WLOG assume you are.  Then your
probability of survival is 1/(N+1).