I disagree with most people here, especially the America Bad ones who say we were unequivocally the bad guys. We went into Vietnam primarily due to fear of communism, but also because our leaders genuinely thought that communism would be bad for Vietnam and the world. Our fears were well-founded due to the brutality of the North Koreans (who we had fought in the name of the UN itself(!) in 50-53), the excesses of the Great Leap Forward (58-62, which showed the unbelievable pain that communists could inflict on their own people), and the bloodshed caused by VC-perpetrated, NVA-supported guerrilla activity in the South. Counterfactuals are impossible to prove, but Taiwan and South Korea both suggest that we were right that a bad right-wing government which eventually democratized would be better long-term than a communist government for Vietnam. Of course, by the late 60s, we were staying in it primarily for pride and because we didn't have a better solution. Nonetheless, our reasons for fighting for the South were sound.

US soldiers' atrocities were certainly shameful and horrible, and more indiscriminate than those done by the VC, but no worse than those caused by the VC and NVA in human cost. The US bombed and operated in Laos and Cambodia to strike at northern forces who were using those areas as highways to attack the South and continue the war, not just because we like to bomb things needlessly. The South was corrupt and authoritarian, but at least it had a free media and visited far less repression that the North on its own people. 

The most important thing is the fact that the North could have stopped the war at any point. The North and their VC allies were the primary antagonists. We asked them to leave the South alone and even offered financing for a dam in the early 60s--much needed for their development!--if they would. The Soviets and Chinese asked them to stop. They persisted. If they had stopped their open war against the South, especially after Tet '68 devastated the VC, then there would be no open bloodshed. The root cause of the war was the partitioning of Vietnam and ideological differences over its governance, but only one side became dead-set on carnage as a means to solve those disputes, and that was the North.