He isn't reborn when he becomes Labour leader, and what he says and thought are relevant. In exactly the same way we rightly criticise Theresa May's disastrous spell in charge of the Home Office and didn't wipe the slate clean when she became leader. Could you imagine if in the early days of her job as PM a Tory voter on here had tried to make the argument along the lines of: 'well she hasn't shown a nasty authoritarian streak and attempted to abolish human rights because someone had a cat since she became PM so we can say that's in the past'? They'd have been laughed at and rightly so.
Besides, if the argument is that he's intelligent enough not to hang around with anti-semites, be in anti-semitic facebook groups, laud terrorists, or like anti-semitic art now that he's leader, then the argument explodes the favoured defence of him as somehow an extraordinarily unlucky man who repeatedly happened, through no fault of his own, to be repeatedly drawn in to these controversies. It suggests rather that he did know what the people he was associating with were doing and saying and thought that no impediment to being associated with them.
And for all the talk of Corbyn 'being determined to speak to both sides' I can't seem to recall much or any evidence of him speaking alongside controversial speakers from the other side of this debate.