One thing is that Mai Lai was a massacre, plain and simple, it wasn't something that was planned that way, more rampaging troops and poor discipline along with pent up stress etc. That's a war crime. Its as bad as when the Germans butchered the population of a French village in WW2 for supporting the Resistance.
Hiroshima and Nagasaki actually saved lives and quite possibly Japan and Japanese culture, as horrific as that sounds. If we'd not have nuked them, and instead gone ahead with Olympic/Downfall then you'd have another 8 - 12 months of heavy firebombing combined with a total sea blockade with mines, subs and repeated air sweeps from Carrier launched planes.
Prior to the nuking of the two cities the Japanese were BARELY getting along with minimal rationing, and their supply situation was close to collapse. A tighter blockade (and it would have gotten tighter, the USAF was mining coastal waters and reaping a fearsome toll of small craft) would have seen this supply chain collapse entirely and then you're throwing the country into a famine.
Also the Japanese were still killing anywhere between 30 - 75000 people a MONTH in their occupied territories (mainly to take food etc so they could survive) with a 'conservative' estimate being somewhere around 40k people dying in Japanese occupied territory every month and this WOULD go up if Japan fell into the grip of a famine. So lets go with that to start with an have say 9 months of Japan in famine with the 'conservative' estimate of 40k people dying outside of Japan a month.
Add to this a ramped up firebombing of Japan and constant air attacks from carrier planes.
Now comes Olympic/Downfall the invasion of Japan. The Japanese were not dumb and had figured out where the Allied forces would land (mainly because there's very few suitable zones) and were in the process of massively fortifying the area.
They were also training up a militia and they would have at best been armed with bolt action rifles, or bamboo spears, and their main anti-tank 'weapon' was strapping satchel charges to themselves, and hoping that when they ran at a tank they were not A. cut in half by machine gun fire, and B, when it ran them over, the charge detonated. And they were planning something on the order of 2 million or so of these reservists who would have been thrown at the US/Allied forces when they landed. So you're looking at something far far worse than anything the US had encountered in the Pacific island assaults thus far.
Yes the Japanese were massing aircraft but I doubt these would have been that effective mainly due to piss poor pilot training and massive air sweeps in the build up to the invasion attacking airbases. But the US was still planning to take a simply unheard of level of casualties (well not Eastern front level but unheard of for them).
The Japanese General staff even had ideas of releasing bio weapons. Not on the US troops, but their own people to both slow down the US forces trying to help them (if they did at all) or help spread the infections to them. This includes Small pox and Bubonic plague. They also were looking at using gas on the beaches. But then again so was the US IF the Japanese did first.
You're looking at hundreds of thousands of allied casualties (dead and wounded) and MILLIONS of dead Japanese. Not to mention the ongoing deaths in their conquered territories.
And then you've the risk of a Soviet invasion of the North, which would result in Japan becoming another Korea/Vietnam situation. I think 'ol Bombs Away LeMay's prophecy of 'Japanese only being a language spoken in hell' would be true. The US forces could have used the nukes against other cities (and they planned to and were producing more nukes to drop for the Olympic invasion).
So the white hot flash and annihilation of two cities and tens of thousands of people actually, perversly saved the lives of Millions, and probably the culture of Japan.