It would be hard to find people who weren't (by today's standards) back then.
People had a hard time believing the stories until they actually saw the gas chambers and bodies (or photographs of them).
There had been a lot of atrocity stories about Germany conduct in Belgium in the First World War that hadn't been true, so people were skeptical about the rumors.
A classic comment of the day occured when Jan Karski delivered his report about Auschwitz:
Supreme Court Justice Felix Frankfurter, a Jew, had told Karski: Young man, I cant believe you. Asked if he doubted Karski, Frankfurter replied: You dont understand me. I didnt say he isnt telling the truth, I said I cant believe him.
Germans were technical advisers to the Turks in their genocide of Armenian Christians during World War I.
Despite first-hand reports, skepticism prevailed there too.