Do you think the invasion of Nazi Germany was collective punishment and a horrible crime against humanity?
No.
But the bombing of German cities, with a deliberate aim of destroying worker's housing, certainly was.
Bomber Harris wasn't hanged, because his side won, not because he wasn't a war criminal.
And his strategy likely lengthened the war, and cost thousands of allied soldiers their lives, in addition to its being a criminal collective punishment against the German people.
The criminal 'strategic' bombing of Germany was caused partly by a desire for a way to strike back against an inaccessible enemy; And partly by a bizzare doctrine that held that bombing civillians
lowered their will to continue the fight - a doctrine that was observably the exact
opposite of the truth, and yet which remains a heartfelt belief to many even today.
As anyone who had toured the bombed out homes of Londoners could attest, you really cannot bomb people into submission. What you get from the survivors is
defiance.
Crimes against humanity certainly
also occurred in the ground war, once the incasion of Europe began; But these were largely localised and small scale crimes by individual company commanders, or at even smaller scales, and some were even prosecuted.