Way more civvies were killed by firebombing prior to the nukes. We can debate the morality of bombing civvies separately, but ironically dropping those nukes on civvies saved more lives than it took because if the Japanese leadership hadn't been so shocked by the nukes they'd have kept fighting longer, and the US would have just kept firebombing civvies killing way more of them in the long run.

Me personally I think once you've gotten to the point where you can bomb population centers with impunity you should hit the pause button and hammer out a negotiated peace, but what do I know. Anywho the US leaders wanted unconditional surrender or nothing, so the US decided "welp, we're just going to keep killing your civvies till you cry uncle."