If you accept as fact that the opposition and protests against the war forced the U.S. government to withdraw American troops from Viet Nam and stop funding South Viet Nam's government then would not those who opposed and protested be responsible for the millions of Cambodians and South Vietnamese civilians who were killed after the war by the Khmer Rouge and North Vietnamese due to the withdrawal of protection we provided for those civilians?
Just as, by standing by and allowing the massacres to happen, we bear some responsibility for the atrocities in Rwanda. We, as a powerful democratic country, had the ability to act and refrained from doing so.
Lots of mitigating factors of course. Allowing evil to happen is not the same as doing evil yourself. The responsible use of power requires the knowledge of power's limitations: you can't impose peace.
In this imperfect world, we are often called on to choose the least of several evils. Usually the best way to deal with such a situation is to behave in such a way that the situation never arises in the first place. But that requires both luck and better judgement than most of us have, individually or collectively.