Unfortunately, the following criticism is Corbyn's own fault.
Corbyn showed just as little appetite to defend Williamson as he has previously demonstrated in relation to Jackie Walker, Ken Livingstone, Marc Wadsworth and others targeted for slander and removal by the Blairites.
Corbyn has proved once again that he will do whatever is necessary to maintain unity with his party’s right-wing, while proving he can be trusted to form a government that loyally serves the interests of the British ruling class. The initial victims of Corbyn’s political cowardice and treachery have been his own supporters. If he ever becomes prime minister, those who will pay the price will be the millions of workers and young people who have looked to him to wage a struggle against austerity, war and the growing assault on basic democratic rights. [Source]
Furthermore, Corbyn's weakness, his inability, or unwillingness, to rally the youth in favor of the human rights of Palestinians (many of whom are very young), is a very bad sign and a clear indication as to why he isn't already Prime Minister.