Actually there is one famous Sherman order for murder. General Sherman to U.S. Brigadier General Louis Douglass Watkins at Calhoun, Georgia, on Oct. 29, 1864:
Can you not send over to Fairmount and Adairsville, burn 10 or 12 houses of known secessionists, kill a few at random and let them know it will be repeated every time a train is fired upon from Resaca to Kingston.
Trains were legitimate targets of war.
Well, somebody in Sherman's army tolerated rape. From William Gilmore Simms:
We have been told of successful outrages of this unmentionable character being practiced on women dwelling in the suburbs. Many are understood to have taken place in remote country settlements, and two cases are described where young negresses were brutally forced by the wretches and afterwards murdered -- one of them being thrust, when half dead, head down, into a mud puddle, and there held until she was suffocated. ... Regiments, in successive relays, subjected scores of these poor women [rb: black women in this case] to the torture of their embraces ...
I bet that put a damper on the train snipers.
Not by non-uniformed combatants, unless you're willing to suffer the consequences.
You Southron types are fond of dragging out quotes from "Gangs of New York" or "Outlaw Josey Wales" so I won't feel bad about offering a quote from that marvelous Australian movie "Breaker Morant". The scene is the court room where Major Thomas is cross-examining the first witness. The witness is talking about how he had to discipline one of the defendants for putting prisoners in open carriages in front of trains:
Major Thomas: Tell me, Mr. Robertson what was Lt Hancock's reason for putting Boer prisoners on open cattle cars on the trains.
Capt. Robertson: Well the Boers had been mining the lines and blowing up a lot of trains. He thought it might stop them.
Major Thomas: Well did it?
So the question really isn't so much whether Sherman shot guerrillas or whether they were in uniform. It's whether the attacks on the trains stopped. Did they?