Don't quote me out of context.

In the context of 1945, after everything the US had been through and everything they were about to go through to get to unconditional surrender. Again, to them, it was just a big bomb, there was no quasi-religious anti-nuclear cult aspect at the time. Killed fewer people than fire-bombing in Tokyo.

Today it would be clearly illegal and non-proportional to use nukes in response to a much smaller conventional attack, in addition to whatever philosophical concerns you might have.

But worth pointing out that a mass-casualty event on the scale of 9/11 actually did cross the line, a nuclear response would have been legal, assuming you could identify and target the guilty party. Not that international "legal" even matters at that point, US policy is deliberately strategically ambiguous. You do not have to wait until somebody nukes you first.