Police are trained to shoot to kill. Malice aforethought means he had the intent to kill.
http://definitions.uslegal.com/m/malice-aforethought/
Malice aforethought is the the deliberate intent to cause death or great bodily harm to another person before a person commits the crime. Malice aforethought is an element that must be proved in the crime of first degree murder. This description of the perpetrator's state of mind basically means that he or she had an intent to inflict injury without legal justification or excuse (legal justification included such defenses as self-defense, while excuse includes mental illness and duress).
Malice aforethought is comprised of any one of the following three elements: (1) an intent to kill; (2) an intent to inflict grievous bodily injury; or (3) an intent to act in a manner that creates a plain and strong likelihood that death or grievous harm will follow. Of these three prongs of malice, the first two prongs require a specific intent on the part of the defendant, measured subjectively, while the third prong only requires a general intent, measured both subjectively and objectively. Accordingly, malice aforethought may exist without an actual intent to kill or do grievous bodily harm, if there is proof of the "third prong" of malice. This simply means that the perpetrator knew of circumstances that a reasonably prudent person would have known created a plain and strong likelihood of death or grievous bodily harm resulting from the perpetrator's act. The law can infer malice from circumstantial evidence, such as from the intentional use of a deadly weapon.