The horrors of guerrilla warfare were well known and studied even in that era. I have read post-war comments by Marse Lee to the effect that guerrilla warfare always devolves to reprisals and atrocities, with “women and priests hiding bombs in their skirts.” Spain and the Peninsular Wars being the recent example they studied.
They knew that GW makes direct targets of noncombatants, leading to massacres of civilians, concentration camps etc. Lee felt an honorable defeat and national reconciliation was preferable to decades of dirty war, atrocities, and the much deeper national hatreds that would be developed. It’s been a few years since I read that in a CW1 history, don’t quote me!
I've no doubt your quote is good; I've seen similar. But my point was, he was quoted as having changed his mind privately by 1870, when he unbosomed himself of his private thoughts to a visiting ex-Confederate from Texas. That man confided in a minister and seminary dean he knew, who in turn published the recollection in his memoirs, near the end of the century.
The confidential conversation with Lee took place at the close of a meeting called by the U.S. Army of the surviving senior Confederate generals. General Rosecrans presided, and was charged with reporting back what the Confederate generals were thinking. This was at the time the Force Acts were under active consideration.