I'm staunchly opposed to Trump, and I believed, even before the last election began in earnest, that Hillary was a bad candidate. In addition to the reasons I dislike Hillary, from a "bad candidate" perspective, she isn't tremendously charismatic, and she comes off as aloof.
I don't dislike her because of Benghazi or the email scandal (both of which, were largely politically driven and given much more attention than they warranted), but because she is a corrupt establishment politician. I still (reluctantly) voted for her, because I knew that Trump would be far worse for America (while politicians tend to be corrupt, Trump's corruption is on a different scale altogether), and as election day approached, it seemed like it was going to be a very tight race. The fact that she was an establishment politician plays into why she was also a bad candidate in 2016, as, on both sides of the aisle, people were looking for an outsider. It was clear to me during 2016 that Bernie generated much more enthusiasm among Democrats than Hillary did, and he also didn't elicit the same level of hatred that the right had for Hillary - Hillary created more enthusiasm for the Republicans in their opposition to her than she did for Democrats in supporting her.