From what I'm hearing (and reading) the Democratic debate last night was a disaster. Hillary Clinton is almost certainly going to be the Democratic nominee, but she also looks like the weaker general election candidate (and as this has gone on, weaker Democratic primary candidate). There are perfectly logical reasons for that.
Clinton is a mainstream Democrat. She is actually to the right of most of the Democratic base, and that should position her well for a general election run. This year, however, being mainstream is a big negative. Outside of the DC political class, most people across the spectrum are not happy with the way the government is [not] working right now, and mainstream candidates are symbols of the broken system. For Republicans that means Trump and Cruz are dominant. For Democrats, who are far more diverse (demographically and with respect to their disposition) don't much go for crazy, but there is a large group of left leaning voters who want to see someone different. I suspect they would have gone ape shit for Warren, but Sanders is right there for most of them.
Aside from insider/outsider appearance, there is also the actual policy positions of Americans vs. historical party positions. Most Americans would have loved to see the big banks broken up and (ir)responsible CEO's forced to pay back their obscene salaries if not outright jailed. Clinton took huge speaking fees from them. Now pretty much everyone (at least everyone with a remotely coherent thought on the subject) knows the Iraq war was a immense mistake. Clinton voted for it, yes, but more recently she has been reliably more hawkish than many other Democrats, and far more so than the party base. These are big issues, and the former is even a big issue to Republican voters who are more likely to see Clinton as friendly to big banks than Trump (not true, but that's going to be the perception).
There is also--obviously and wrongly--a huge anti-Hillary bias among much of the population. They're mostly Republicans, but lots of independents and even a fair number of Democrats too and those latter groups are a danger: if it's Hillary vs. Trump they're the most likely group to switch sides or stay home.
Then there is the demographic issue that doesn't help Clinton come the general election. Her supporters are older, have voted regularly, and are almost certainly going to vote in November for the Democrat on the ballot. Sanders' supporters skew young, with far more who have never voted before, and they are excited by Bernie himself. If he is not on the ballot in November there is a much better chance that more of his supporters will just stay home. Clinton might not need them (especially if Cruz is the nominee) but with her other general election negatives it would definitely be better to have them.
At this point in the contest, Bernie Sanders should be helping line up his supporters to vote for the Democratic nominee in November, and continuing attacks on Hillary Clinton isn't going to do that. Clinton also needs to make nice with Sanders supporters and her going after Sanders himself isn't going to do that. This debate seems like one that should have taken place 2+ months ago, while those earlier debates which were far more warm and fuzzy should have been the one last night.