While each of these actions must be subject to vigorous, sustained challenges—in the courts, through legislation, and in the streets—it is essential that at the top of the list is Mr. Trump’s decision to release violent criminals who attacked police and ransacked the Capitol. Many of them, including leaders of the Proud Boys and Oath Keepers, are flaunting their newfound freedom, ominously pledging retribution against those who investigated, prosecuted, and imprisoned them.