When Mr. Trump mused about withdrawing the United States from the NATO alliance in 2018, Mr. Bolton was reportedly distressed and rallied to keep it from happening. And, in questioning fellow Republican Jon Huntsman’s decision to serve as ambassador to China in President Barack Obama’s administration in 2011, Mr. Bolton said, “There is no patriotic obligation to help advance the career of a politician who is otherwise pursuing interests that are fundamentally antithetical to your values.” In other words, Mr. Trump’s frequent demeaning of the Atlantic alliance, his obtuse bromance with Putin, and his apparent acquiescence in Russian interference with the American electoral process may have persuaded Mr. Bolton to desert the president on principle.

Then there are his professional principles. Mr. Bolton, unlike Mr. Trump and some of the fiercest members of his inner circle, is a seasoned government professional with an informed respect for the institutional architecture and ethos of American foreign policy. Before becoming Mr. Trump’s national security adviser, Mr. Bolton served as ambassador to the United Nations, undersecretary of state, assistant secretary of state and assistant attorney general.

Mr. Bolton reportedly characterized Mr. Trump’s meddling with aid to Ukraine as a “drug deal” — a crude metaphor for actions that violate his sense of foreign policy professionalism. He also disdained the president’s circumvention of normal diplomatic channels by informally enlisting Rudolph Giuliani, his personal lawyer, whom Mr. Bolton called a “hand grenade who’s going to blow everybody up.” Separate from his sense of patriotic duty, Mr. Bolton may have felt that Mr. Trump had so demeaned the integrity of the foreign policy structure that something radical had to be done.

Well, maybe. Another explanation is personal indignation and greed. Mr. Bolton spent much of his career dreaming of the national security adviser job, and reportedly lobbied the president for it for years. And, of course, his book is due to come out March 17, and these revelations are sure to make it an instant best seller (a fact not lost on the president: Mr. Trump’s backers have predictably cast him as a “disgruntled” former employee, and Mr. Trump himself has accused him of merely trying to sell books).

Let’s not judge John Bolton too harshly, though. He lasted almost a year and a half in a job under a famously mercurial president, and toward the end was reportedly unhappy in it. And his book, for which he received a reported $2 million advance, didn’t need this revelation to make it a hot item or line his pockets. So while I’m sure Mr. Bolton doesn’t mind a taste of revenge and higher book sales, in all likelihood the two more honorable factors feature more heavily in Mr. Bolton’s decision-making.