This is a great, thoughtful post. Also the tweet you quote is, quite clearly, a snarky joke and not meant to be taken literally. Was interested in this aspect of what you say:

If I'm rude to you and you shoot me, you will probably go to jail. If I'm rude to a cop and he shoots me, he can easily falsify his report to avoid jailtime. Or some other aspect of his position as an officer will help get in the way of justice.

My partner is a defense barrister, and the ways in which the decks are stacked against (often black) defendants is incredible, really. Judges are often white, out of touch old men and have bias against black culture that borders on the absurd - one of our friends was discussing a trial the other day in which the defendant was said to be using "gang vernacular" for saying the word "fam" to describe his friend. Furthermore, if you have two defendants, they are held separately in custody and have to provide separate statements, and any inconsistency is pored over by the prosecution. What do police officers do if there is an allegation of excessive force against them? They go to the canteen and work together on drafting identical statements, and they close ranks around people they work with. That's not some big, secret revelation - that's literally how things are done, and the judiciary accept these submissions because they are deferential towards the profession because, hey, why would the police lie about something? They're professionals!