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.
https://en.wikipedia.org/wiki/Value_pluralism
And modern psychology and neuroscience appear to back it up.
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.
"Absolute" is the key word.
Hegel was full of shit.
One thing a trusted member of your group can do is call you out on bad behavior, rather than supporting it.
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.
But in the small we are always compromising on some things in favor of others. A principle can be upheld at great cost until it finally gives way. One principle can crumble away in favor of others very suddenly.