A good answer to this is found in the book The Lost Indictment of Robert E. Lee: The Forgotten Case against an American Icon by John Reeves. "While Lee is held in high esteem by many Americans nowadays," writes Reeves, "the same could not be said in April 1865. Politicians, soldiers, and ordinary people were much more divided in their opinion of his role in the bloody Civil War." In the North, "one of the most constant demands that emerged from the national discussion on treason and traitors was that Lee was foremost among those deserving of punishment".

Reeves points to comments made by several Northern activists and newspaper editorials. Abolitionist Wendell Phillips called Lee the "bloodiest and guiltiest" of all the rebels and he was fit to be hanged. Frederick Douglass compared Lee to John Wilkes Booth, saying Booth "was not one whit guiltier" than Lee was for what Lee had done. The New York Tribune agreed, writing that "such a man is more guilty than any other". The Cleveland Morning Dealer compared Lee to the "arch-traitor" Jefferson Davis, arguing Lee was "the wickeder and baser of the two" because it was "a selfish ambition that led him, Judas-like to betray his country". The Ohio Farmer said the U.S. government should "give him ten feet of rope, and six feet of soil," adding that "every traitor earned this reward" and Lee more than all.

Harper's Weekly criticized Lee's farewell address to his troops, in which he had expressed admiration to them for their "consciousness of duty" and for "devotion to your Country" (the implication being their "Country" was the Confederate States of America). The magazine said this showed Lee was unapologetic, praising soldiers who "would have destroyed this nation". William Lloyd Garrison wrote the farewell address was a "slap in the face" to loyal soldiers. The New York Post wrote that the farewell letter showed that Northern apologists of Lee had been "foolishly" deceived into thinking he was a man of "higher and nobler" character than Jefferson Davis: "the letter in question shows him as bitter and graceless a traitor as Davis himself":

"...[T]his rebel general, this deserter from the old flag, who ran away with a lie in his mouth, and entered the general service of the Confederate army when he had promised his sister to serve only his State—this person cannot refrain, even when he has given up the contest as hopeless, to blurt out his treason, and to use his influence over the army which he had surrendered, to rouse them to new rebellion and justify future insurrections. If any loyal man began latterly, on false reports, to think well of this Lee, he has now, in the letter from which we have quoted, the materials for a revision of his judgment."

The New York Times concurred, calling Lee's farewell address defiant against the U.S. government, inspired by "a contemptuous, sullen and persistent disregard of any obligation of loyalty".

The New York Times editorial also criticized Lee's portraits taken by Matthew Brady, shortly after Lee's surrender at Appomattox. These were, no less, taken on the day Lee was notified of Lincoln's assassination—Reeves makes a plausible case they were taken immediately after Lee had been informed of the assassination, since the man who arranged the photo session is also the man who is supposed to have informed Lee of the assassination. Yet, even in these circumstances, having surrendered and knowing of Lincoln's death, Lee chose to pose for the photos in his full Confederate military uniform. The New York Times criticized this as beneath "the breeding of a gentleman" as Lee boasted to be, showing the trait of an undisciplined soldier. They called it distasteful for Lee to be shown "in the toggery [garments] of the bastard Confederacy", which the newspaper believed had been done for its "political significance".

The Providence Journal mocked the photo session, which had produced six photographs "all very life-like" showing Lee "in six different attitudes". In one, the paper wrote, Lee appeared in a "meditative mood", as he must have felt before resigning from the U.S. Army; in another, he appeared in a "sneaking mood...having determined to become a traitor to his country...sneaking away from Washington"; and in another, he appeared chivalric, "permitting the Union prisoners to be starved". The last, mocked the Providence Journal, depicted Lee responding to the cheering crowds as he directed "the United States officers to remove the the colored sentinels from his private mansion, as they were quite offensive to him and his wife".

In a later editorial, the New York Times argued that Robert E. Lee should be brought to trial for treason, and the surrender terms he had agreed to with Gen. Ulysses S. Grant permitted this:

"Gen. Lee can be brought to trial for treason, without hindrance, either legal or honorary. Ought he to be? Most decidedly. He has 'levied war against the United States' more strenuously than any other man in the land, and thereby has been specially guilty of the crime of treason, as defined in the Constitution of the United States."

Reeves states the Chicago Tribune published a similar editorial, calling for Lee to be tried for treason which he describes was even harsher than the Times editorial, though Reeves does not quote from it (and I could not find it online).

The Illinois State Journal also concurred, stating that President Andrew Johnson's proclamation on who would be prosecuted and who wouldn't be should include Lee, regardless of the surrender terms. They began their editorial with:

We observe on the part of some of our contemporaries a disposition, while generally expressing a hope that the ring leaders in the rebellion will be brought to punishment, to compliment and single out General Robert E. Lee as a wonderful pattern of a hero, and a magnificent specimen of a chivalrous and magnanimous foe. We are willing to accord to that General the meed of praise to which ability and bravery in a bad cause may entitle him; but we protest against these attempts to canonize him, or represent him as anything more than a bold rebel, whose reputation and conceded abilities enabled him to do more for the cause of treason, and against the Government, than meaner and more insignificant agents of a bad cause could do...[A]t a most critical moment, [Lee] basely deserted and betrayed the Government which had educated and supported him, and between two days made his way into the camp of treason—a blot which will forever cling to his name—his cruel conduct towards our prisoners confined in the prison pens of Richmond, was such that, instead of empty compliments for magnanimity, he is deserving of the execration of the civilized world for his studied inhumanity and barbarism."

There was some sympathy for Robert E. Lee in the North, at least on the point of whether or not he should be tried for treason, given the surrender terms. The issue largely fell on partisan lines. Democrat-affiliated newspapers often said he should not be tried, while Republican-affiliated papers said he should. "True to their old sympathies," wrote the New York Times, "anti-administration journals oppose any civil procedure against Robert E. Lee. Their plea is that the terms of his surrender exempted him from all further interference, civil as well as military."

One such newspaper was the Louisville Journal, stating they "cannot believe the report" that Lee was going to be indicted and tried for treason, "because such an action against General Lee by the United States authority would be an utter violation of the articles of surrender which he...accepted from General Grant. The faith of the nation was sacredly pledged by those articles, and they amply protect him".

(cont'd...)