Morally corrupt because he'll say anything in order to win power.
KS got elected head of the Labour Party on a platform of being both respectable, competent and leftish. As head of the Labour Party, he has overseen a vicious purge against even vaguely left. He has reneged on just about every pledge he has made. He said that Israel had the right to cut off power and water -- then tried to deny it when it didn't play well.
I'm still going to be voting Labour because the Tories' corruption is far far worse -- the reason the Spectator focusses on dullness is because they know the Tories are far worse vis a vis the corruption stakes. But let's be clear about who we're dealing with here.