The allies fought Germany because they had no choice - the Nazis basically declared war on the world. Yet the Holocaust is very much a post-facto justification. At the time, the allies did nothing to impede the Holocaust, and it played very little part in their justifications (just watch 'Why we fight' and check where it's mentioned - nowhere).

Only after WW2 the allies decided to remember the Holocaust existed, and justice still played a second fiddle to interests.