Actually, when the Rosenberg case was going on, liberals marched in demonstrations to support them.
Dallas Dunlap

Two points, Dallas:

Lee was genuinely caught in a serious dilemma. That I will grant you.

He took an oath as a military officer and he broke that oath. He then took up arms against his old comrades. That is being a traitor. It’s what the word means. He did this as honorably as a man could under the circumstances, but it does not affect the issue of treason. The term refers to actions, not motives, and many who have committed treason have done so for what they believed were honorable motives. And sometimes they were right and the cause they betrayed was wrong. But not this time.

At the time of secession that North had done nothing aggressive towards the South. Slavery was not going to spread, and those Americans who opposed slavery were going to be politically more powerful than those who supported it. In the words of Sam Houston, Texas’s first governor, opposed secession:

What is there that is free that we have not? Are our rights invaded and no Government ready to protect them? No! Are our institutions wrested from us and others foreign to our taste forced upon us? No! Is the right of free speech, a free press, or free suffrage taken from us? No! Has our property been taken from us and the Government failed to interpose when called upon? No, none of these! The rights of the States and the rights of individuals are still maintained. We have yet the Constitution, we have yet a judiciary, which has never been appealed to in vain — we have yet just laws and officers to administer them; and an army and navy, ready to maintain any and every constitutional right of the citizen. Whence then this clamor about disunion?”

There is genuine tragedy in Lee’s story, but that does not negate the fact that he broke his oath, took up arms, and joined a cause that killed hundreds of thousands, a cause that humankind almost unanimously regards as bad.

My second point is that sovereign states play no role in the constitution. The idea is an add-on to support special pleading by slavers. They had no compunctions about enforcing the Fugitive Slave Law earlier on unwilling Northerners. As James Madison explained in some detail, there was neither state nor national sovereignty, only popular sovereignty. Governments were not sovereign, they existed to serve the people.

As Madison well knew and discussed, there is a tension between states and national government that could only be kept manageable if no side pushed its position as far as possible. Compromise is inherent in the constitution’s logic. The tragedy here is that as slavery became increasingly profitable, that portion of the country increasingly distanced itself from the principles of the American Revolution, until there was so little common ground that mutually acceptable compromise was impossible.

If there is a deeper lesson here, it is that our constitution can work only when there is enough mutual respect by contending parties to prefer compromise over seeking to destroy the other side, or leaving. That broke down in 1860 and I fear it is breaking down today.