I think the big issue is the religious / political environment. I read somewhere an explanation, which pretty much said you cannot criticise Israel government policy (e.g. dropping bombs on civilians, letting your police murder civilians) without being accused of antisemitism, as Israel consider the state/government and the religion is a single entity. Because Corbyn thinks it is wrong for Israel to take land from the Palestinians by force, build a wall, then kill any Palestinians that protest, he is declared antisemitic, and therefore put in the same group as Hitler and the Third Reich.
It's all just political games designed to undermine the opposition, and put doubt into the minds of the public. It is hard to not be influenced by the propaganda and spin. Of course, some people are antisemitic too!