Of course the world is not black and white, there are shades of grey. That is completely orthogonal to subjectivity vs objectivity. Just because a situation is grey and no one can agree on which shade of grey it is, doesn't mean the situation doesn't objectively have a shade. It can just mean humans are fallible and can't see perfectly, so we're all wrong to some degree about the correct shade. But there could still be a correct shade.
Separately, every single example you brought up is black-and-white. If a "knife-wielding maniac" were in the process of killing random innocents and police don't have a way of nonlethally restraining them, then of course the police are justified in killing them. If a cop in the USA or anywhere was in mortal danger and had no way of nonlethally restraining their attacker, then killing their attacker is justified regardless of the mental health of their attacker. If an innocent isn't following contradictory commands and isn't threatening anyone's life, of course it isn't justified to kill them, what are you talking about?! There is nothing grey about the fact that failing to follow contradictory commands by police should clearly not be punishable by death?!?!!
I'm sure there are tons of Han Chinese on the street who think concentration camps for Muslims in China is acceptable, just like there are tons of Americans on the street who think concentration camps for Muslims in America is acceptable, just like there were tons of Americans on the street who thought that internment camps for Japanese-Americans were acceptable, tons of Palestinians who think all Jews are invaders who should be wiped out, tons of Jews who think all Palestinians are suicide-bombers who should be wiped out.
Those people are wrong. Those examples are not shades of grey and not subjective, lots of people disagree because lots of people are wrong.
I'm also wrong about lots of things, I should not be dictator of the world, and neither should anyone else. That's why we need a system with rules set up so that the more wrong ideas shrink in influence and the less wrong ideas spread in influence.