There is only one explanation for the president’s relentless attacks. He thought that Mueller was likely to throw the book at him. And there are only two explanations for that expectation. Either Trump knew he deserved it, or Trump assumed Mueller would sink to his own level of mendacity and self-serving to pervert justice. The idea that a public servant, indeed, a team of public servants, would quietly discharge a mission with honor was utterly beyond Trump’s fathoming.
The country had an unpleasant job that needed doing. The president of United States had surrounded himself with people who lied about their contacts with highly placed Russians. He had fired the director of the FBI, James B. Comey, and within hours he personally assured the Russian ambassador that he did so to shut down an inquiry into these lies. It was possible all this could be explained as the product of incompetence and naivete, because Trump had been utterly unprepared for the presidency and was surrounded by gangsters and clowns. But it was also possible something intentional was going on.
Someone had to sort out the facts. The task would be exhausting, it would be thankless, and it would likely end in some degree of vilification.