There is a really important difference, that you may be missing. The use of/need for the attacks on Hiroshima and Nakasaki can be argued, and Lord has it been, both here and pretty much anywhere else folks talk about history for extended periods. Any search on the terms here will spew out tons of threads, some very interesting and in great detail.
The difference is when one advocates for mass usage of Special Weapons that did not occur IOTL. That is problematic, depending on exactly how the issue is approached, especially when the advocacy leans into "kill them all, God will know his own" territory.
IIRC, the actual possible plan discussed involved the mass use of tactical nuclear weapons against the PRC, which although quite likely very dumb due to probably not turning out how like the perpetrators would like, is not the same as nuking every major population center in the country (which yeah, is actual mass murder). In the same way that bombing an army, even with high amounts of collateral damage, is probably less of an atrocity than intentionally leveling mostly civilian cities. Which of course Truman, not MacArthur actually did.
In fact, it's possible that MacArthur's chivalry with regards to civilian targets may have cost lives once. Some argue that MacArthur held back on bombing Manila in WW2 due to fear of civilian casualties, which slowed the American advance and gave more time for the retreating IJA to continue engaging in the Rape of Manila. I wonder if that incident influenced MacArthur's attitudes in Korea.