You can only have one absolute moral principle; everything else must ultimately be contingent on not violating that core principle.
I am usually bringing this up on HN in the context of free speech, because I think free speech is a poor choice to make your absolute moral principle.
In this context, there's another example of a poor choice for an absolute principle.
Brotherhood, fraternity, loyalty to your group is frequently a good thing. Many things only work with trust.
But this is what it looks like when brotherhood -- loyalty to your fellow police officers, in this case -- is your absolute moral principle. Upholding the law and protecting the innocent come second to protecting your own.
To me, the cops quitting in solidarity is a far worse problem than the cops pushing the old man.
We need to grasp that in a national eruption of 1000's of interactions, some of them will be bad. There will be emotions, stupidity, even racism and true bad acting. I fully expect that even in a highly professional and well-trained police force ... that stupid will happen.
BUT - the cops quitting ... this is 1) not a decision made 'in the moment of passion in the blink of an eye' and 2) as you say, it arguably contradicts the very nature of their oath.
My cousin, a Marine, said to me that a common creed is 'Unit, Corps, God, Country'. I don't know if that's official, colloquial, or even widely true ... but ... I found it really deeply wrong to put 'unit and corps' above 'god and country'. But I never got the chance to discuss it with him.
I'm pretty sure "unit corps God country" is from A Few Good Men, and the Marine motto is God Country Corps, which makes more sense.
That minor point aside, I agree with you. The "bad apples" defense doesn't stand up to the mass resignation in support of these cops.