To be nice to your accusers is therefore a norm derived from systems lacking due process.
No. It's a recognition that even if you stack due process to the moon, you still have the machinery operated by fallible humans. And I say this with explicit and direct experience from the criminal justice system.
I'm an anarchist that abhors hierarchy and authority, and yet here I am standing up when some self-important goober wearing a silly-looking robe walks into the room. I don't do this for my own mother, and I love my mom. I call the judge "Your Honor" at the end of every complete sentence, and I talk to them in the third person by referring to them as an institution "The Court" rather than an individual.
I hate everything about this song and dance, but I do the pirouette because my ultimate goal is to advocate for my clients. The goal isn't dying on petty hills.
By definition, my clientele tends to select from a segment of the population with higher than average impulsivity. A big part of my job is euphemistically referred to as "client control", and the majority of that is making sure they never talk. A client of mine yelling in court to a judge "THIS IS FUCKED UP" after they just deny a release motion (this has happened, of course) might release some catharsis and feel stellar in the immediate moment, but goddamn does it harpoon my ability to convince the judge in the future that you are a safe bet to release to the public.
Is it unfair that individuals get scrutinized under extremely unfavorable and stressful conditions? Absolutely! Maybe you've never committed a violent felony (so far), but can you really blame someone for having an outburst after some government official sentences them to months in a cage? Add the fact that you're handcuffed, and attired in a humiliatingly large orange smock and also wearing Crocs for shoes.
Which hills are worth dying over? Kieran probably felt a lot of gratification from being a complete asshole to people in power. So now what? He's still kicked out of school. He's sinking thousands of dollars in legal fees to sue the school, and his conduct has given the judge multiple legitimate legal reasons to dismiss his case, but also personal bias reasons to dismiss his case too (Public interest law firms like the ACLU spend a looooong time "Plaintiff Shopping" for a reason) The latter is undeniably unfair, there's no disputing that. But to recall one of the adages I rely on too often when talking to my clients: "Do you want to be right or do you want to win?"
People should be aware when pursuing a goal will come at the expense of another (potentially more important) goal. If they push forth regardless, it's difficult to muster up sympathy for their decision.