There are ways in which we do that. Our supervisors are not only reviewing or reports, but also body camera footage and in car videos. Their supervisors also review it and above them, a Use of Force Unit and Internal Affairs. The whole "I feared for my life," by itself doesn't stand up in grand jury anymore and hasn't for awhile, everything is taken into account, as in the totality of the circumstances.
I'm sure there are way departments do review what happened, but it's clearly not very good across the board. Also, as mentioned several times in this thread, police unions are incredibly overpowered at keeping bad cops in uniform and on the payroll. But what I was alluding to was that fellow officers should keep other fellow officers in check and the culture of "well they're my superior" should be axed immediately. In my line of work, if my lead, manager, or director were doing something to put patient data at risk, I'd say something, in fact I have said something, more than once. Is it easy to do? Not at all but sometimes it's needed.
Cops could also quit trying to stand behind other cops that violate the law or overstep their boundaries. Here in Salt Lake City, we had a detective who tried to force a nurse to draw a blood sample from a badly burned accident victim without a warrant. When the nurse refused, because an unauthorized blood draw is illegal, she was arrested. The detective was fired from the police force, but immediately hired elsewhere because of course "he did nothing wrong". That detective should not be able to ever work as a cop again if he can't understand something as simple as "you need a warrant for that".
As for the whole, "I feared for my life" not holding up to a grand jury, forgive me, but that's absolutely not true. There are plenty of news stories out there covering the phenomena of cops getting away with crimes too:
Example 1,
Example 2,
Example 3.
Philando Castile was shot by Jeronimo Yanez after Castile told him he had a legal, licensed firearm in the car with him. I've been to classes, I know that if you're pulled over and carrying a weapon you let the cop know immediately. They're supposed to disarm you if they feel unsafe. What did Yanez do? Shot Castile five times at nearly point-blank range after never attempting to disarm Castile. The outcome? Yanez walked and was given nearly $50k to leave the department all because he feared for his life when Castile was simply getting his wallet.
Terence Crutcher was shot by Betty Jo Shelby because he didn't know what was happening to his vehicle that was inoperable in the middle of the road. Was he on drugs? The autopsy said yes, but did that mean he deserved to be shot? Absolutely not. Crutcher was even tased by another officer before Shelby shot him all because she feared for her life. Shelby's punishment? Acquittal and she was rehired on another police force in Rogers County, Oklahoma.
Freddie Gray was literally killed while in custody, as in arrested and in the back of a transport van. He died either due to excessive force by the officers or outright neglect. The outcome? Three officers were acquitted and three had the charges dropped.
These are just a few well-known examples and I'm sure there are hundreds, if not thousands more.
Something closer to home that just happened is the killing of Bernardo Palacios-Carbajal. While Palacios-Carbajal did have a gun, he was shot 20 times in the back while running from police. Do you know what's not endangering your life? A suspect running away from you. I feel like in this instance a taser would've been the right call.
Here's the bodycam footage of what happened,
it does involve someone being repeatedly shot so I'll put it in spoilers:
Seriously, 20 times in the back with several of the shots landing after the suspect is on the ground and clearly incapacitated or dead? If that doesn't scream abuse of power because they "feared for their lives" I'm not sure what does. And you know what? I'd put money on it that these officers walk without any form of disciplinary action.