There is no such thing as a 'warning shot' nor do you shoot anyone in the legs.
The requirement is that you give a verbal warning "in a loud and clear voice" IF there is enough time. If the suspect is behaving in way that leads you to think he will endanger your life or that of anyone nearby, then you may open fire "without warning" If and when you do open fire always "aim at the centre of the exposed part of the body" For example: the chest if they are standing in full view, or their head if that is all you can see.
The parts in quotes are from the Green Card - Rules of Engagement for military personnel.
There was a 'scandal' in the UK press a while back about a so-called 'shoot to kill policy' in Northern Ireland. It was stupid - Of course it is shoot to kill, there is no middle ground here. If you are are dealing with terrorists or suspected terrorists, there is no time to consider your options, you have to react faster than they do.
'Batman' should count himself very lucky to be alive right now.....