As I recall, the "baby killer" statements and spitting on returning GIs stemmed from this:
http://en.wikipedia.org/wiki/My_Lai_Massacre
That phrase lived on for many years. My Lai was most likely a situation where the GIs cracked. The heat, bugs, the dysentery the unrelenting harassment by the NVA and the VC working for you in the day and mortaring you at night. Sometimes all this can break loose. Some broke and they had sick morons in command, so the pack mentality prevailed and it became a riot.
The Soldier in combat will cross a line that most people never face, once you have crossed that line it cannot be undone, guys on the ground told me the first one was bad, then it got easier.
If you read the history of the incident you see the best and the worst of us.
The scout copter driver that saw what was happening and put his feeble craft between the civilians and the rioters. The Huey crews that heard him call and came down and turned their guns toward the rioters while others evacuated whomever they could save.
Whenever you have a combat operation in progress around a civilian population civilians will be killed and maimed. When providing air support you can take out a structure where bad guys are, but there could be innocent people in there too, but you take it out anyway to protect the men on the ground. Life in those situations is not uncomplicated.