The Clintons are proponents of the "Third Way" ideology represented by Democrats who came to power during the 80s and 90s. They represent a break with the older (socialist) traditions of FDR's New Deal and Johnson's Great Society. The Third Way ideologues had a new willingness to take money from corporate interests, and were effective in creating a coalition amongst well heeled Democrats and corporate elites by focusing on social / wedge issues instead of economics issues.
In many ways, they adopted the supply side/trickle down/ austerity politics of Reagan. These policies are now largely seen to have failed, as have many others that the Clintons and other conservative Democrats have championed (the Drug War, austerity policies, free trade deals written to favor corporations over workers, wars, use of racist language to appeal to conservatives). For years, the left was angry about these rightward moves of the party establishment, but accepted the lesser of two evils argument.
For decades, there were no credible Democratic primary alternatives to the ruling Third Way ideology. Working class voters and progressives began to drift away from the party that failed to fight for their interests, and they called themselves independents.
The failed policies of the last thirty years have produced a public that is restless and looking for a change, and for the first time in decades, it has an alternative in the person of Bernie Sanders. Hillary Clinton is a less expert politician than her husband or Barrack Obama, and is slightly to the right on them on many issues. All of these factors together have put her in the position of inexpertly defending failed Third Way policies. Because of Sanders, she's been put in a position of defending right wing positions she's championed her whole life and attacking dearly held ideas of the left. When she angrily said that single payer healthcare will "never, ever" come to pass, she came across as a Republican. Her embrace of the war criminal Henry Kissinger, her advocacy of war, her taking of money from all the bête noires of the left have made it obvious that their party has been taken over by a right wing movement.
Now, against that backdrop her positioning to the left seems incredibly opportunistic, and in fact, like a lie. It is one thing to have a mole in your ranks. It is another to have one who fools people with her transparent misdirections. Watch the "Hillary lying for 13 minutes straight" video with this frame in mind, and you'll understand why people are so angry.