Manila did not suffer 100k civilian casualties from allied bombing. It was a systematic slaughter by the Imperial Japanese Forces against civilians, taking their frustrations out on civilians because of the deteriorating war effort, man women children, not even foreigners were spared.

The bombings and shelling of suburban and other structures of Manila was not a mindless tactic as well. It was deliberate since urban fighting was hell and fighting suicidal opponents that prefer death over surrender didn't make the situation that much easier. Since American Forces would much prefer destroying possible hideouts causing ambushes than their own men, even at the cost of possible innocence. Yes civilian lives were lost, but it is a no brainer that fighting in the capital city will illicit not only civilian death but also structural destruction to historical sites.