Why did guerrilla warfare anger the US troops Vietnam war?

For the same reasons it angered British troops when they fought Americans during the Revolutionary War (1775-1783); the British considered guerrilla warfare to be "dirty fighting", "sneaky fighting", "unfair", "un-gentlemanly", and "cowardly."

Guerilla warfare, Submarine warfare, and military snipers all were considered: Dirty Fighting; Sneaky Fighting; Unfair; Ungentlemanly; and Cowardly.

For the 21st century all of those "Dirty Forms of Warfare" are now considered the new wave of doing things (they are popular with today's generation of people).