Trying to come up with ways to stop police killings is treating the symptom, not the cause. It's a pretty complicated issue that won't be solved on a web forum, but as long as the centurions view the public as an enemy threat, and as long as the citizens view the centurions as enemy oppressors, ambushes and the killing of unarmed people will continue, just as it has from the earliest days of humanity.
When first responders respond first with deadly force, to shoot first and ask questions later, tit-for-tat incidents are both proportional and expected. People can be policed only to the extent to which they are willing to be policed. Enlisting and obtaining the cooperation and trust of the public is paramount to effective policing. Compliance by force is the exact wrong way to enlist that cooperation and trust.