Possibly because they had seen the concentration camps.
MIdnight Run's claim that Eisenhower 'murdered' a million POWs is based on the book "Other Losses" by James Bacque, a Canadian writer. Not all historians agree with his findings.
Midnight Run/Dave's sentence:
"It was part of the same disrespect that the Allies held for Germany throughout the war, like when they were murdering innocent people in Dresden."
is rather odd. As Germany was the enemy, and the extent of Hitler's atrocities was becoming known, it is hard to see why the Allies should have respected them. He also seems unable to accept that Hitler's bombs killed innocent people (presumably he means civilians) just as dead as those of the Allies did. Hitler began his war on Poland by bombing four non military targets with no air defences, and smashed Warsaw into surrender. He blitzed London and other major cities in Britain, and carried out the Baedeker Raids on non-miilitary targets with the deliberate intention of frightening the British in surrender.