Yeah, right, blame the people who tried to warn you about all the flaws of your oh-so-perfect candidate and who were proven entirely correct by this election's outcome. 
#HillbotLogic
Whilst I'm no fan of Hillary, surely the hyperbole of the more rabid opponents of Hillary on the left (who, ironically, seemed to pick up and use anti-Hillary tropes that originated on the right) regarding how corrupt she was contributed significantly to her being a flawed candidate? I mean, I think she was a bad candidate, but I think her predicament was made significantly worse by the image of her as this ultra-corrupt right-wing Democrat that was crafted during the primary season, with significant input from elements of the pro-Sanders left.
I doubt that Clinton's critics on the left would have been quiet even with Sanders out of the contest; if anything, Clinton is fortunate that Sanders was there to bring some of the less belligerent Clinton skeptics on the left more firmly into the Democratic coalition rather than getting preoccupied with third-party organizing that would have posed a much larger problem for Clinton's candidacy. Clinton's share of the vote among Democrats didn't drop significantly compared to Obama, and the voters who failed to match 2008-2012 turnout were from groups that tended to align with Clinton during the primaries, so I'm not sure where the thrust of this criticism is even supposed to fall.
Simply that the biggest drag for Clinton in the election campaign seemed to be the perception of her (correct or not) as dishonest and corrupt, personified most in her long-lasting travails over her emails, and to a lesser extent her connections to Wall Street and wealthy donors. My take, such as it is, has always been that this perception of her wasn't a particularly significant problem until the primary season got underway in late 2015, when the Sanders camp began using it in their critiques of her. Given that Sanders got such a huge amount of exposure for a candidate comparatively far to the left of the Democratic party's mainstream, this had the knock on effect of helping the perception of her as dishonest and corrupt seep into the conscious of the general public who were perhaps not following the election closely but nonetheless had vague views on the major candidates. Of course, you can't blame the Sanders camp for this solely; the media played a key role in popularising this image of Clinton, the Republicans piled in too and Clinton herself didn't do a good job of refuting these allegations (which in my view were often more than mere allegations). Moreover, as you noted Sanders came out to bat for Clinton very strongly in the election campaign which probably helped avoid bigger losses to non-voting/other candidates. However, by that point the damage was already done to her image. To be clear, I don't think the anti-Clinton left were particularly responsible for her defeat, but they did play a part in her campaign's death by a thousand cuts.
