Laws and norms, ethics and principles? Meaningless concepts. In Mr. Trump’s world, nothing is sacred and everything is transactional. This is a man who is so much more interested in winning than in governing that he filed for re-election the same day he was inaugurated. It’s the core of who he is, as almost anyone who has had even a glancing dealing with him over the years — especially in real estate, where nothing matters but the sale — will attest. If you help him or say nice things about him, you’re the best. If you get in his way or criticize him, you’re unfair, dishonest, terrible. Failing, even.

Ask “Liddle” Bob Corker, the Tennessee senator who earned Mr. Trump’s outrage by criticizing the Republican tax bill, then seemed to get back on the president’s good side with a few empathetic words. Or Senator John McCain, who is dealing with terminal brain cancer even as Mr. Trump keeps mocking him for his vote last year against repealing Obamacare. Or Attorney General Jeff Sessions, who was Mr. Trump’s first and most ardent supporter in the Senate, but who has secured a spot in the president’s doghouse after recusing himself from the Russia investigation. On Wednesday, Mr. Trump fired yet another broadside at Mr. Sessions, complaining that his failure to investigate alleged abuses of the Foreign Intelligence Surveillance Court is “disgraceful” — in all caps, naturally.

None of this is remotely surprising anymore, but even so, Mr. Trump’s tweet about Judge Curiel’s border-wall decision revealed once again the depths of the president’s own self-interest and his utter lack of principles. When this is the driving force of the commander in chief, and it is aimed at our governmental and social institutions, it is profoundly destructive.

Shortly after the 2016 rally where he disparaged the competence, integrity and ethnicity of Judge Curiel, Mr. Trump was asked why he did not just settle the Trump University case and be done with it. After predicting that he would win outright, he said: “I could settle that case. … I don’t want to settle the case. Because you know what? Because I’m a man of principle.” (In November 2016, days after he won the election, Mr. Trump settled, eventually paying $25 million.)

Fortunately, as this tale reveals, there are in fact still people of principle in public service, and they’re doing important work every day to preserve our institutions and counteract this president’s worst impulses.

Judge Curiel, for one, has continued to do his job, carefully applying the law to the cases that come before him, no matter how obnoxious the litigants might be. In his ruling on Tuesday, the judge acknowledged the “heated political debate” surrounding the border wall, and quoted a passage on the role of courts by Chief Justice John Roberts Jr. — who, he slyly noted, is a “fellow Indiana native”: “Courts are vested with the authority to interpret the law; we possess neither the expertise nor the prerogative to make policy judgments. Those decisions are entrusted to our nation’s elected leaders, who can be thrown out of office if the people disagree with them. It is not our job to protect the people from the consequences of their political choices.”

That is what being a public servant in America sounds like, and it requires a level of selflessness and devotion to democratic ideals that is alien to Mr. Trump.