All IMO of course;
Personally I think the police involved had to make a decision that no-one here would be willing / able to make (I stand to be corrected of course).
The guy was wearing very suspicious clothing for the weather (especially the temperatures on the tube), and refused to obey police instructions. The police then had to treat him as a potential suicide bomber and kill him (yes it may sound extreme but shooting someone in the leg doesn't work like it does in the movies). They were juggling the life of one man against the possibility of hundreds more - I mean FFS the guy ran away from Police onto a tube train!
If the policeman who shot him had been right and he HAD been carrying a bomb he would be an absolute hero, unflinching, willing to carry out his duty without hesitation etc etc etc.
Just because a tragedy has occured does not necessarily mean someone is to blame. With the current environment in this country and London in particular this sort of thing is bound to happen - the police are trying to protect thousands of people in London (me being one of them), against an enemy they can't see until it's often too late.
It's bad, it's tragic, it's sad etc etc etc... but is it the policeman's fault? Certainly not.