Some would argue that HE is humane, none could argue that a firestorm
How can one form of killing be described as humane compared with another?
It may be true to say that sudden death from HE is better than a slower death from oxygen starvation. OTOH a slow death from crush injury or being entombed under rubble may be less preferable to sudden incineration from a firestorm.
On the carpet bombing in Japan, I have a vague recollection that they used a lot of cottage industry rather than larger factories for some things so a wide bombing was necessary.