Since I can only base my observations on the video posted above, I will respond.
1) The cop in this case has his bodycam recording prior to his exit of vehicle. I am guessing it is mounted on his sternum, based upon the angle. This is most common.
2) At 10 seconds he is exiting his vehicle and at 11 seconds we see Davis is closing in and at 12 seconds appears to be almost even with the back tires. Officer is trying to scream commands.
3) At 17 seconds the attack has moved away from the patrol car and into the intersection. Officer is yelling drop the knife. The officer is facing directly into the sun.
4) Officer continues to try to create distance by tactically retreating. At the 20 second mark of this video there appears to be a flashlight in Davis' left hand and an object which can easily be construed to be a knife in his right. Davis continues to close in on the officer and refuses to "drop the knife" (or whatever it is). Because the officer is moving while being under what he believes to be an attack, part of Davis' body goes out of the bodycam field of view at the 21 second mark.
5) By the 23 second mark the officer has his weapon out and begins shooting. By the 25 second mark the officer has hit Davis who is now on the ground. By 53 seconds he has called for medics.
The officer in a space of 43 seconds did not have time to turn his camera on or off. Awful angles are due to location of the camera on his torso and the cop moving trying to save his own life. As to a cop screaming something, doesn't make it true-that can be true. We hire fallible people in every profession. People do make mistakes. However in this case it appears to me that the cop went from stepping out of his car to "oh ****" in a space of less than one second. In reading the article, it states that the object was a knife. And the investigation continues.
Body cams to help a lot. They aren't perfect but they are a damned useful tool. And it appears to me that the officer used his camera in the manner it was intended to be used.