It was done to avoid more casualties, merciful deed it was
Belgrade was bombed three times during WWII,
first time is was done by Axis forces in spring 1941 when people didn't want to accept agreement with Third Reich signed by Yugoslav government.
Second and third bombings happened near the end of war by Allies. British and Murican forces bombed Belgrade on orthodox easter 1944. Second time Muricans bombed Belgrade in September of same year.
Fourth time Belgrade was bombed in same period of the year (spring) and third time by Muricans in 20th century was when NATO bombed Belgrade in 1999. to overthrow Milosevic. Beat that
What interests me, does Americans have any idea in case of their demise from world's number one power and creation of some war crimes court which will have to process every crime done in last 100 years or maybe even more, how much guilt they will have to deal with and how severe the punishment would be for their actions? Hypothetically speaking of course.
I don't think there was ever a nation in history that managed to do so many crimes against other nations in such short period of time. Germans and Japanese don't even come close. Even Soviets are left in the dust.