There is no such thing as a 'Shoot to Kill' policy.
The rules state that you may open fire if there is a threat to you, or the people you are protecting.
You must give a warning, if there is time to do so.
You may NOT aim to wound - it is clearly laid out that you 'aim for the exposed part of the target'
Basically - If you can see all of them, then aim for the centre of the body, if you can only see their head, then aim for the centre of their head.
The requirement is to STOP the threat, and if that means killing, then that is what will happen, but nowhere does it state 'shoot to kill' in the Green Card (Rules of Engagement)
There is a lot of stuff in there about giving three warnings and such, but the fact is that all of these decisions are made in seconds or less, and there is no way back once committed. If the guy HAD been a bomber, and got onto the train killing another 20-30 people, then everyone would have been condemning the police for being too slow.
I no longer have a Green Card, but you can be certain that those policemen would have followed the rules laid down there....