Thirteen Reasons Why has enough betrayals to fill up a daytime soap opera. Loyalty is noticeably absent from this story. Characters – at least as they're depicted by Clay and Hannah – rarely get past each other's reputations and surface attributes. Tony and Clay do develop a sense of loyalty to Hannah after listening to the tapes, but by then it's too late. By the end of the tapes, it's clear that Hannah feels betrayed by everyone around her: peers, parents, and teachers. But it's important to remember that Hannah has betrayed herself, too. She has sex with Bryce, the guy who allegedly raped Jessica and chronically abuses his peers. This (and other) acts of self-betrayal seem even more harmful to Hannah than the betrayals she experiences at the hands of others.
Hannah ultimately takes her own life because she can't even be loyal to herself.
Hannah's decision not to step in when she thinks Bryce is raping Jessica, and her failure to help Jessica afterwards, is the most serious betrayal in the book.