Stop making things up. He's quite obviously NOT talking about the entire South. He's talking about "the petulant and persistent Secessionists." And the comparison is apt.
Nothing at all wrong with that.
Seems to me he's got the right idea.
He's talking about waging a war. He's absolutely right about what the law of war allowed. You seem confused over whether in a war an army must observe the same niceties that prevail in peacetime.
Wrong.
The "viz." is not in what Sherman actually wrote.
Here's a screen capture of the message in the OR [Series I, Vol. 32, Part II, p. 280]:
View attachment 95292
As can be clearly seen, the "viz." is not a part of the message and was added by someone else to try to make it seem Sherman was saying something he wasn't. It's a typical tactic we see with neoconfederate websites, which have to rely on deceit to overcome the fact that history doesn't support their falsehoods.
If we look at the passage in context, we see the false doctrine is that "the Southern planters say that they made the South, that they own it, and that they can do as they please, even to break up our government and shut up the natural avenues of trade, intercourse and commerce."
Notice the semicolon after the phrase, "for which we as a people are responsible." It means he's making another thought.
So he believes the war to result from a false political doctrine; he believes that all [white] people have the right to self-government, and he would give all a chance to reflect and when in error to recant.
Poor analysis. Sherman was speaking of a wartime situation, and he's right. By the laws of warfare, the army had all the rights he said they had.