Alex, there's a fuller explanation in a link posted above, but, essentially, the strategy is that he is more likely to survive if he leaves the two better shots alive. Their best strategy would then have to be to shoot at one another and the great probability is that one of them would be killed, leaving the poor shooter with only one target for which he has a 50% chance. If he shoots to kill on the first shot and is lucky, then the chance of his death on the second shot is either 80% or 100%, depending on who he killed.