Unarmed civilians, often of color, being killed by police is nothing new. But it is only recently that the stories are getting more attention from the public and the media. The stories often cause outrage when it is reported that the police officers who caused the killings are not charged. People wonder how this can be if the victim was unarmed?
The answer is that police officers are legally permitted to use deadly force if they perceive a threat, regardless as to whether or not the threat is real. Therefore, if a police officer reasonably believes that his or her life, or the life of another person, is in danger, then deadly force is permissible under the law.
In addition to "the defense of life" standard being a legally permissible circumstance in which an officer can use deadly force, an officer can also use deadly force to stop a suspect from escaping if the officer has probable cause to think that the suspect poses a dangerous threat to the public. And again, it doesn't matter if the suspect actually is dangerous, just that the officer reasonably believes this.
To take it a step further, the reasonableness standard is based on "objective reasonableness," which asks if the officer was acting reasonable based on the unique circumstances and how other officers would act in the same situation. Objective reasonableness is also fluid and changes as the circumstances change.
Therefore, an officer might be legally permitted to use deadly force in one minute, but not the next if the situation, or his understanding of the situation, changes. For example, an officer who sees something in a suspect's hand that he believes is a gun may be permitted to use deadly force (depending on other circumstances), but if the officer then realizes that the item is not a gun but instead a hairbrush, deadly force is no longer permissible.
These standards were developed by the U.S. Supreme Court in the 1980s and are the focus of many police brutality lawsuits. But, as Vox reports, activists are now broadening the issue and asking if policy should be focused on preventing the use of deadly force and not just trying to justify it after the fact.
You can read more here on that topic.