This article and video explain that police officers customarily and sensibly wear holsters designed for maximum control of their firearms. That makes it impossible for an officer's gun to fall out of a holster or be misappropriated by someone else. Further safety features address specific parts of the gun such as the slide and trigger, again to prevent it from firing accidentally or under hostile action. In order to the remove and fire the gun, it is necessary to know the exact method which the holster is set to permit, in which case the officer can draw and fire quite easily. Without the knowledge and practice of how this holster works, the gun is not coming lose, because the technology is quite well designed. Responsible gun owners use equipment like this to avoid stupid accidents like shooting themselves in the crotch or having children fire a gun. Concerned about your safety? You could bolt one of these to your bed and not have to worry about shooting yourself in your sleep, yet still have your weapon ready at hand.
None of this does any good if the officer is paranoid and delusional. By paranoid, I mean generally inclined to believe that everyone is out to get him when it is not the case. By delusional, I mean believing something which not only did not happen, but
could not happen as claimed.
There is equipment designed to protect officers and other people around them. Regrettably it cannot overcome all human failings. If officers do not understand their equipment, use it properly, and
trust that it will perform as designed -- then they can't take advantage of the safety features as intended.
Put these two factors together and the result is a loss of life which was entirely preventable. This is just one incident in a widespread pattern of police brutality marked not just by physical violence, but by deep psychological distortions. When one officer makes a mistake which unnecessarily kills a civilian, that person needs to find a different line of work. And when the same problem occurs repeatedly throughout multiple departments, then it indicates a pervasive flaw in law enforcement as a whole.
Of course, another consideration is that parts of this problem are not mistakes but are desired goals of a police state. That's a political issue and somewhat different.