What I really come away from the impression veterans and civilians of the time give of, is simply that it was unsporting, undignified, and unfair.
That, much more than it was punishingly cruel, in excess of death and dismemberment by other means.
They say, yeah, it's okay to zip a supersonic bullet through soft tissue such that it shreds with cavitation, and smash it through bone that splinters and explodes, so people lose whole limbs. And that's okay. It's like a deer hunt.
But hey, you don't hunt animals by mutilating their lings with bug spray. That's no fair. You wouldn't consume an elk carcass tainted with persistent organic pollutants. No fair. We can't even bury our dead.
But, you know what? You aren't going to bury your dead, when they're incinerated by high explosives.
And hey, what's fair about war at all? What's fair about several hundred planes unloading kilotons of conventional munitions, indiscriminately, to break an apparatus of geopolitics.
Gee, unfair, undignified, unsportsmanlike, in the pain, injury and slow death (leaving a contaminated corpse) department.
When you piece those explicit details together, the real problem is that it deals a degree of harm to the perception of gallantry, duty and honor of service. It's pretty much impossible to pat a chemical burn on the back, and say "there, there, thanks for defending the country" but, you can still pull that off, when sharing the room with an amputee that lost a limb to ballistics.
Exchanging chemical weapons for aerial bombardment exhibits "differences" but nothing that makes me say "wow, one is okay, and the other isn't."
Both are essentially equally bad, and neither is remotely good, or an improvement over the other.
People have their preferences, but in the same way that they rationalize the side effect of rampant smog and pollution as bad, but millions of fatal car accidents as normal, and a calculated risk.
The real choice being put on the table though, is: Would you choose between an air war that includes both regular explosions and chemical attacks, or just a regular air war only?
At that point, the choice becomes more rational. We say: "Hmmm, well, at least I can take some of the destruction out of the equation. I get to make one kind of misery go away. I can choose X = A as an alternative to X = (A + B + C). A never goes away, and nothing can be done about that, but at least I don't have to think about B and C anymore."
So, it's not about incediaries and high explosives being okay. Or aerial bombardment being different, and better. It's just that they can't be taken off the table, under any circumstances, no matter whatever else gets negotiated. I guess it's just a fundamental characteristic of destruction, that if destroying things is on the table, the basics of destruction get to keep their scalar qualities. You can limit the method of destruction but not the quantity.