Given that everyone who was in-country from the spring of 1969 to 1972 was keenly aware that U.S. troops were being withdrawn from Vietnam, it is nothing short of a miracle that morale remained as good as it did (which wasn't very) during this period. It was precisely during this period that the overwhelming majority of homicides occurred. But it must be taken into account that the soldiers who were sent to Vietnam during this period, especially the draftees, had been bombarded for years by the anti-war movement and were more inclined to question authority, especially military authority.
It was also no help to good order and discipline when judges started giving convicted criminals the choice of jail or the Army. You can be assured that even an understrength rifle platoon would have preferred to remain understrength than to be given sociopaths as replacements. This misguided policy on the part of state judiciary systems was part of the reason that discipline began to erode from about 1970 onwards. A good number of the intentional homicides committed during this period were perpetrated by these sociopaths.
Finally, it must be understood that intentional homicides, especially of superior officers and non-commissioned officers, have occurred in every war in history. This includes Americans in WWII and Korea.