It was a good thing.

"I like President Truman, the way Dad would of wanted me to. He dropped the atom bomb on Japan and saved millions of lives because if he hadn't of, then there would of been a lot more war than there was and more people would of been killed. I think it was a good thing to drop the atomic bomb on Japan." (Moore, end of Chapter 6, paper by Walter Kovacs from Charlton Home).

This quote shows one theme of the book; killing (or hurting) some to save more. Veidt's plan is the culmination of this theme, the epitome of it. He kills almost all of New York City, in order to prevent a World War from happening and to promote world peace. Rorschach says that Truman did the same thing with the atom bomb. It is ironic that he would say that he likes Truman and is glad he did this, because at the end of the novel when Veidt has launched his plan, Rorschach says that he should not be able to get away with it, that it is wrong.