Although they were warlike, the Assyrians were also quite a cultured bunch. Think of the library at Nineveh and the relief of the lion hunt of Assurbanipal in the British Museum.
And lest we forget, Genghis Khan is also well known for his religious tolerance and there is an account of Western monks ending up in a debate between the religions held at his court.
These were rough dudes. You are probably right that they would have strained at the quality of mercy required to imagine their enemies as worth saving from nuclear reduction.
But they were from a different era of world history.
I agree that the US has been a relatively benign imperial power.
But I think it lost a lot of lustre as a result of the rendition and torture of some of its prisoners.
You should also note that the US did actually use its atomic bombs against the Japanese! Only nuclear power actually to explode one in a war.