International law forbids the direct killing of civilians. But if a civilian works in a factory that makes guns used to kill the enemy can that make the civilian a legitimate target?
Hitler came to power because he had enough support of the people. In fact he was very popular in the beginning of his political power, even abroad. He still got a lot of support when he started the war. So one could say that the German population was at least complicit at waging war. On the other hand they were not able to reverse their decision. Another problem is, not everyone voted for Hitler.
Civilians and soldiers are both human beings. The soldier knows what to do in wartime and the civilians will back their soldier's actions as long as they themselves are not put in harms way. So, as long as everyting goes well they (most) don't care. If on the other hand things go bad they stop backing their soldiers.
There is no comparison between the bombing of Dresden and the use of the A-bombs. There is a comparison though between the bombing of Dresden and Tokyo. Dresden and Tokyo destruction didn't shorten the war, the A-bombs did.
My opinion is that the bombing of Dresden is not a war crime but it comes very close and that monument should have been build soon after the war not now.