You're arguing from hindsight, Metchosin.

If the police was sure from the onset they had to prevent a serious incident their acting was lousy. But that's not how it goes.

- A suspect (only reason for the suspicion: the address) leaves the house. Let's follow him to find other suspects or a hideout.
He enters a (perhaps quite empty) bus: Let's follow him
- He has a bulky jacket: He could be transporting a bomb or be on a suicide mission or just so: Let's follow him and watch closely. Meanwhile phone to get permission to shoot, just in case.
- He goes to a tube station (more people around than in the bus): It's getting more risky, let's challenge him to find out whether he is a terrorist or innocent.
When challenged he runs and jumps a barrier: Then and only then with seconds to go the idea 'we have to act immediately' comes in their minds and they act as trained.

With him not starting running he'd be still alive. But from his position I also can understand why he started running. There are a lot of hypothetical scenarios in which running and jumping was the best action. Unfortunately, in the actual case it wasn't. If he had had the idea they wanted to check him for a bomb he'd stopped, smiled at them and very slowly opened his jacket.

Two (groups of) persons acting with no knowledge what was in the others mind(s). Acting wrongly and quickly based on false hypotheses.

Wolfgang