The thing is the cop comes on a scene with people standing around and a woman lunging after another with the knife which is attempted murder. If someone was defending themselves because their life was threatened, I doubt they would go after them with the knife when a police officer had showed up.
The cop was left with little choice but to shoot the person with the knife about to stab someone. Maybe in a perfect world he could have grabbed his taser to stop her and bring her down but when arriving on the scene were a stabbing was called in or something like that, I cannot blame the cop for what he did. Whoever was the bad girl in the story, the cop killed someone attempting to stab someone and saved the other person's life.
If the potential stabee was doing something illegal to stabber, then she can still go to prison but makes it hard to punish the cop for his actions. In any other time it would just be seen as a tragic end to a call.