Was the United States justified in using the atomic bomb to end the war with Japan?

Yes

indeed it was a terrible weapon. But according to research (Done by yours truly) thousands of American Soldiers and Marines would have died. To drop the bomb was a far better alternative than to send a massive invading force to take over Japan. The casualties would amount to over 200,000 armed members. Japan, would likely fight to the brutal end. It was better this way. To probably prevent a longer war. And more people dead.