I'm not Jewish but live close to a large Jewish population and worked with many people from that population over a long period of time. Labour has been the party supported by the majority of Jewish people. Over several years, concerns have been expressed within the Jewish community locally about anti semitism in the Labour party. None of my Jewish friends are anti Palestinian and they're uncomfortable about the current government etc. This doesn't mean they don't have long memories about Germany in the 1930's which is when Hitler was (according to Ken) a Zionist.
How anyone can see Hitler's desire to rid Germany of its Jews as Zionist baffles me. 1938 saw Kristalnicht, and the arrival in the Uk of many Jewish children from Vienna, Germany etc - they were the lucky children with parents who had financial means and could get them out before the holocaust. One of my close friends lost every single member of his extended family, that experience isn't confined to only one person is it.
If Ken doesn't realise that he's offended many Jewish people, he lacks the emotional sensitivity gene. The survivors of the holocaust are now few but their families remain. The holocaust was directed at Roma people, those with disabilities, homosexuals, communists and other so called minority groups. I can't imagine Ken would feel comfortable making comments that could be seen as offensive towards those groups.
Tom Watson described him as drunk on his own infamy.