It appears to be a terrible, tragic mistake. [I am qualifying this since the investigation is incomplete and there is not yet an answer to why Menezes acted so suspiciously.]
At the same time the UK and the West cannot let this mistake change the approach to preventing terror. Captain's Quarters puts it in perspective:
Menezes, a Catholic, had legally emigrated to Britain three years earlier and worked as an electrician. He spoke English well and would have understood the commands to stop, and his family says he had no reason to flee from the police -- and yet he did.
Many people will take this time to second-guess the London police and British special services. They will note the tragic consequences of a shoot-first policy that killed an apparently innocent man just trying to get to work, although one would expect that an innocent man would have stopped when commanded to do so instead of running for the nearest subway car. The police themselves will now second-guess themselves when it comes to making split-second decisions that could mean death in either direction.
Debate on tactics has its place and its benefits, but when such debate comes, it has to take place in the proper context -- and that context is the war which Islamofascist terrorists have declared on the West.