other politicians, particularly those in the GOP, have gleefully attacked Hilary for a decade or more. if Sanders does not -- no word game here -- that makes him decidedly different.
Clinton has been promoting the idea that he's "about to go negative!" in NY, trying desperately to get people not to like him. her campaign is making false accusations like this, and like saying he agrees with Trump's sexist remarks, lying about his voting record, or claiming that he's singling her out over taking superpac money when he's been vocally against all superpacs practically since they were invented, while at the same time basically plagiarizing his stump speeches because he's actually not like 'every other politician.'
if he was like everyone else, he'd be attacking her all the time, always talking about her FBI investigation and her husband and her pantsuits.
if he was like every other politician, he wouldn't have such a consistent record. he wouldn't be so darned honest. he for sure would have distanced himself from the word 'socialist' since it causes so much misunderstanding.
Sanders has nothing to 'fear' about attacking Hilary -- there are an awful lot of very legitimate, negative things he could say. the thing is i guess, he seems to have a lot of other things he'd rather say while he's finally getting some attention.