There's a lot of sexism at play but I think a lot of Clinton hate comes from a genuine distaste for 90s style politics being propagated by someone who simply is not a good politician. This looks worse in a post-Obama climate: she comes off as stale and bloodless, and her speeches rarely impress; people naturally compare her to Obama and are left disappointed.
Things get even uglier when you add an idealistic fringe candidate to the mix. Like Ron Paul, Sanders has very passionate supporters who don't know anything about politics and believe only he can save us from corruption. Everything becomes a purity test in which no politician can possibly match their man's record. Never mind that like Paul, Sanders has little to no accomplishments to speak of (outside of additions to various bills his supporters hate). Hillary Clinton is probably one of the worst candidates to have to deal with a "pure" candidate like that, and I get why his fans are so passionately against her.
And this ties into anger v pragmatism. Obama has achieved quite a bit without being a pure liberal, whereas most politicians who take an all-or-nothing approach don't get anything done. Social Security once only applied to white widows. It took years and decades to make the program what it is today. Likewise I'd expect Obamacare to have a similar history. Yet the typical Sanders fan looks at Obamacare as worthless because it doesn't include [insert item]. Good luck with that type of dogmatism in the real world.
