Cheating is a bit strong of a word. It was more that she colluded with the party to cripple Sanders chances of winning the primary. The party is supposed to be neutral, and not pick favorites during the nomination process. Technically there are no rules saying they have to; in fact, the entire primary process is a show of good faith to their constituents. But the DNC was confirmed through email leaks to break that tradition, and worked to influence the primary process through things like controlling the media narrative and hampering Sanders ability to get his name out there, as well as discrediting him among minority groups. There were also a few more specific allegations about fraudulent behavior with certain states election officials. There's no telling if he would've won otherwise, but it's entirely possible given that he was an incredibly popular candidate. His message was strong with the working class, which is where Hillary fell short tonight...a lot of people think he would've pulled it off.

Don't forget that a member of the DNC Donna Brazile fed multiple (what should have been secret) questions ahead of time to Hillary for a CNN town hall event w/ Bernie.