What happened on the fringes of Sherman's march also happened deep behind union lines in the regions that had fallen early in the war. They happened in many cases with full sanction of the commanding generals. Sherman himself is known to have even ordered his troops to randomly gun down civilians in order to make examples of them...but that is far from the worst of it.
If you want to truly see the worst of the atrocities, look at what close Lincoln associate and Gen. Ben Butler did in occupied New Orleans. He issued an order permitting his soldiers to use the women of the city as prostitutes in the event that they acted in any way that could be construed as insulting or disrespectful. To suggest that Lincoln did not know what Butler was up to (the two were in contact frequently throughout the war and met many times in Washington both before and after the New Orleans event) is absurd.
For an even worse though lesser known case, look at what Gen. Robert Milroy, a Sherman subordinate, did in occupied Tennessee. He literally drafted up murder lists of civilians and sent out death squads to execute them. The lists included names of people who had allegedly supported secession, had given food or shelter to confederates, or who were known to have a son in the confederate armies. Evidence was circumstantial at best and almost entirely based on the word of one man who was a unionist spy with many personal grudges against his neighbors. None of them were ever tried - Milroy simply had them executed on the spot. The executions were often brutal and in clear violation of any rule of law. One of the orders, which contains 58 names, directs the troops to execute the mother of a confederate but do so in a manner to make it appear as if it were an accidental shooting. Another order directs them to arrest a civilian and turn him over to Pittman, who would then be permitted to torture and execute the man as a reward for being an informant. Milroy is also known to have thrown the bodies of his victims into a pond and station soldiers there to guard them for several days, ensuring that the other civilians saw the decomposing corpses. He even ordered some of them executed by hanging with a slip knot which, unlike a noose, causes a drawn out painful death by slow strangulation - a revival of a popular torture method from the middle ages. All of this happened deep behind union lines in territories that the yankees had controlled from the early days of the war. It wasn't anywhere near the fringes of operation and therefore cannot be excused as such.
In war, nobody has a monopoly on virtue.