True, hindsight can color events a bit, though I think there were signs of the genocidal nature (albeit no more and potentially less than was the case with the Soviet Union as well I believe). There was a question I think for the leaders at the time, as to how far Hitler was going to go.

The impression I get is that after the Munich agreement, most countries felt that Germany was going to stop there, so when it then starting making motions towards invading Poland, it signified that Germany just wasn't going to stop, and so had to be stopped sooner or later and sooner likely better.

The somewhat ironic thing is that some people at least felt that the Nazis and Soviets should be left to fight each other, so didn't want to declare war over Poland, but the subsequent partition of Poland based on the Ribbentrop-Molotov pact would have been a bit of a shock to them I think, and made them more wary.