You already said it: a wartime decision, in a war the Brits did not start, against a murderous regime.
Hindsight says Dresden was an unnecessary and unnecessarily brutal attack but General Hindsight was not on duty at the time.
Even without "General Hindsight", such a large and cruel operation had to have some rationale at its time. What was it?
Hiroshima is another story. Soviet troops were defeating Japanese army on the land and America provided Japanese government tangible argument to the population for a speedy capitulation before United States. The division of Germany was not affected by destruction of Dresden (which ended up in Soviet zone) so I wonder what was the real reason.