If we pretend that this is perfectly decent behaviour, how does throwing an anti-Semitic message into it add to the joke? For a lot of people, including myself, the answer to that question is "it doesn't".
As for the comment that you can include hateful comment without yourself expressing those opinions: this was no archive footage of Hitler delivering a speech. This was people holding up a sign with an anti-Semitic message because PewdiePie explicitly asked them to create a sign with an anti-Semitic message and record themselves holding it up.