Few images evoke a more powerful or frightening reaction than that of the assassin. Silently creeping into your room while you sleep, they plunge a pick deep inside your brain, or fill your snoring nostrils with poisonous vapour. Or perhaps skilled fingers from the shadows strike a vital point during mid-step; the coroners say it was a heart attack. Assassins are people to be feared, no doubt. But what do they fear?...