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 can’t believe you.” Asked if he doubted Karski, Frankfurter replied: “You don’t understand me. I didn’t say he isn’t telling the truth, I said I can’t 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.