The aims of terrorists are never easy to understand. But it is reasonable to believe they include the sowing of self-doubt and insecurity. If so, the London bombers have had a victory of sorts.
A society with a tradition of unarmed policing is now coming to terms with the killing by officers of an innocent terrorism suspect in a busy London Tube station. This has added to the sense of shock and unease that Londoners were already feeling after the July 7 suicide bombings and the botched attempt to repeat them last Thursday.
British police are generally considered to be less trigger-happy and to have a more sophisticated approach than their counterparts in many other parts of the world. Few carry guns and the number of times they have been fired in recent years is tiny by global standards.
Anti-terrorism police have been instructed to shoot to kill. But the shooting of an innocent man is a sign of how jittery they have become, at a time when British people are already feeling very nervous.
This is a testing time for a nation facing home-grown terrorism that strikes at its way of life. The range of reaction to the awful death of a young Brazilian electrician on his way to work is an example. Naturally, there are official regrets and apologies, and sympathy for the victim's family. Questions are being raised about a tragic mistake that must be answered.
But the government and much of the public have closed ranks behind the police, sympathising with the officer who fired five shots into the man, and demanding that he not be turned into a scapegoat.
The victim had the misfortune to live at an address that was under surveillance because it was linked to one of the July 7 suicide bombers. He was followed and armed police tried to stop him as he went to buy a rail ticket. He then vaulted a barrier, ran down an escalator and boarded a train, where officers caught him and held him down before he was shot dead.