To paraphrase and reapply Hiko Seijuro: A gun is a weapon. Learning to use a gun is learning to commit murder. Justification is irrelevent.
There are basically two points:
1. Regardless of who you are and who you work for: Never pull a gun unless you intend to fire, if necessary. Never fire a gun unless you intend to kill. Period.
2. The issue with this thread and its subject, for my own local interests (the US) is that the police themselves are no longer what they were meant to be. They are armed individuals working for a paycheck on behalf of the state/city/county who are, in turn, motivated primarily by whatever interest group or corporation will provide them with the most funding. To Protect, and Serve no longer has its intended meaning. The goals of the police should always be to maintain peace, even uncomfortable peace, and to protect the safety of the people and their property from any direct threats, including from their own government.
The travesty that was the police involvement in and against the Occupy movement, as well as other absurd pseudo-militaristic repressive overreaction (i.e. the National Guard at Kent State), just serves as an example of how the police in the US have, in many cases, forgotten the primary purpose of their job. If we can manage to appropriately re-stress the goal of serving and protecting the people, who the government itself is meant to be serving, rather than the government/corporate interests, and be sure that any use of detention or force is only applied where appropriate, then any case in which that force is applied would appropriately provide a justifiable cause for deadly force if ignored.
Basically, if your police are doing their job correctly and honestly in all circumstances, then shooting a fleeing suspect by aiming center mass is ok by me.