Sullivan also noted that Flynn could have been charged with other crimes that were not included in his plea agreement.
"All along, you were an unregistered agent of a foreign country while serving as the national security adviser to the president of the United States," he said. "Arguably, that undermines everything this flag over here stands for. Arguably, you sold your country out."
"I'm not hiding my disgust, my distain, for this criminal offense," he continued.
Sullivan later said he misspoke when suggesting Flynn acted as a foreign agent in the White House, noting Flynn's lobbying work ended before he started working for the Trump administration.
"I felt terrible about that," Sullivan said after a brief recess.
Sullivan said he would take into account Flynn's public service and military career and his cooperation in the investigation when sentencing him. But he underscored the gravity of the offense nevertheless and signaled that would be a major factor.
In a surprise twist, Flynn decided to delay his sentencing on Tuesday, after Sullivan gave him the chance to push it back until his full cooperation in the Mueller investigation and other probes is realized, given that it could result in a more lenient sentence.
Flynn's attorneys said he would do so, citing foreign lobbying charges unveiled Monday against Flynn's former business associate and noting it was likely he would testify at an upcoming trial in that case.
"There's that additional modicum of cooperation that he expects to provide in the [Eastern District of Virginia," Robert Kelner, one of Flynn's attorneys, told Sullivan. "For that reason, we are prepared to take your honor up on the suggestion to delaying sentencing."
Flynn's lawyers ahead of the hearing had suggested their client had been wronged by the FBI agents interviewing him, noting they did not warn him it was a crime to lie to the FBI and that the interview was conducted without White House counsel present.
Flynn's attorneys Robert Kelner and Stephen Anthony clarified Tuesday they did not believe Flynn had been entrapped by the bureau but chose to include details about his interview so the judge could compare Flynn's case to others who have been charged and sentenced for the same crime in connection with Mueller's investigation, arguing he deserved less prison time.
