It's one thing to say "This historical figure has done some pretty awful thing, although they've done many things that deserve to be honored." It's needed perspective, but it doesn't change that those historical figures have great accomplishments that deserve significant honor.
But what accomplishment of the Confederates generals deserve honor? Not their service to the southern cause, because the southern cause is an euphemism for slavery, and no one should get honored for serving slavery. Not victory ; while they won battles here and there, the plain truth is that they lost the battles that mattered the most, and the war with them. Not for their humanism on the battlefield ; people who lead raids to capture free people and sell them as slaves don't deserve to be rewarded for their humane actions.
About the only thing that Lee (and Davis) did that was worthy of anything remotely resembling of honor whatsoever is, ironically, refuse to be honored, and refuse to be involved in Confederate symbolism after the war. Of course, you don't honor that by building statutes to them as Confederate heroes. You honor that by making monuments that emphasize the end of the confederate cause and moving on from it.
At the end of the day, there is just nothing that (most of) these men ever did that warrant putting up statues to them, or keeping them there ; and certainly not in the amount that they exist.
(OTOH, "Perpetual union" and "more perfect union" should be given about as much weight as "Til death do us part", which is to say none at all. The idea of an indivisible political state is loudmouthed nationalist windbaggery, and the "they were traitors" narrative needs to die in a fire already. It's a distraction from the real problem (slavery), and one that only feed into the pro-Confederate efforts to make this about politics and state rights rather than about slavery)