Yes, if it even saved one American life it was worth it imo. War is hell.
The Nagasaki bomb killed several of the allied forces 'prisoners of war'.
Also, Japan's final straw for surrender was the Soviet Union invading them.
Since we're wiki'ing away:
Until August 9, the war council had still insisted on its four conditions for surrender. On that day Hirohito ordered Kido to "quickly control the situation ... because the Soviet Union has declared war against us". He then held an Imperial conference during which he authorized minister Tōgō to notify the Allies that Japan would accept their terms on one condition, that the declaration "does not compromise any demand which prejudices the prerogatives of His Majesty as a Sovereign ruler".
Also:
Throughout the war, Tōgō was among those who doubted that Japan could succeed in a war with the United States. Towards the end, he was one of the chief proponents for acceptance of the Potsdam Declaration which, he felt, contained the best conditions for peace Japan could hope to be offered. Up until the last, he hoped for favorable terms from the Soviet Union. At Tōgō's suggestion, no official response was made to the Declaration at first, though a censored version was released to the Japanese public, while Tōgō waited to hear from Moscow. Unfortunately, many Allied leaders interpreted this silence as a rejection of the Declaration, and so bombing was allowed to continue.
Japan were considering surrender.
So, no, the only thing that was a "necessary evil" was the name of the plane that took the photos of the bombing missions...