Well WW2 is a lot closer in history to us than the American Revolutionary War, and the effects of it are still felt in the horrors that haunt people in their mind every day who had to go through it, the marks left on people's bodies, etc.
Something like the Holocaust was never part of a legitimate war. This wasn't one army against another. It was a deliberate rounding up of people that Hitler didn't like, to murder them and do horrible experiments on them, work them to death, starve them to death, let disease rage through the camps, shoot them, and murder them in gas chambers and burn the bodies in crematoria. That's not war, that's genocide.
When America wanted to break off and form its own country, that's bound to lead to war, and the people were willing to fight. England found out they couldn't keep up a foreign war so far away and so the Americans ended up winning. Of course there were horrible, bloody casualties, but that was the price the Americans were willing to pay to be independent and rule themselves. England in the Revolutionary War wasn't setting out to round up millions of people just to murder them in cold blood and/or torture them. The deaths were part of war.
What the Nazis did is just not comparable because it's so much more evil.