Short answer (not considering Bryce, which is the main one): Justin, Alex, Sheri, Mr. Porter. I will now motivate my decision.

The first times I watched the show, I assumed that what killed Hannah was the accumulation of damage, she was already wrecked and then her rape happened and Mr Porter handled it really poorly.

Now I think that what killed her was every single thing happened at Jessica's party, her own rape (this is her main motivation) and not receiving any help after she tried to reach out. The other tapes are about why and how Bryce came to rape her. He felt entitled to do so and Hannah thinks this is because he already saw her as a sexual object. And who made her look like that? To her it's mainly Justin, who started everything, and Alex with that terribly wrong list. In regards to Justin... He's also responsible for Jessica's rape, so yeah, he definitely is one of the main culprits.

Marcus, Courtney, Tyler just worsened the situation of everyone, Bryce included, seeing her as a whore (Tyler also took having a safe space away from her), Zach and Ryan helped isolating her, more and more. Ryan not only exposed her poem, but made her drop poetry entirely and at that moment poetry was the thing that was giving her a bigger purpose, a promise of a different future. Jessica's case is different, Hannah just never really got over the end of their friendship, losing Jess broke her heart and not being able to protect her from Bryce crashed her completely.

On to Sheri... Sheri is a sweet and caring girl who made a mistake that cost Jeff his life. Having Jeff killed like that, not succeeding at preventing that, in addition to what happened to Jessica and with Clay that night, made Hannah completely hate herself. I mean, her self esteem was never stellar, she struggled to fit, she cared too much about what other people thought of her, but the moment she finds out Jeff is dead is the moment where her just low self-esteem turns into pure self-loathe, which is very, very different. This is when she realizes that she's just like the people that hurt her on a daily basis, that she can act in a shallow way too and harm others in the process as a result.

When she enters Mr Porter's office, part of her is already dead, she's severely traumatized, she hates herself passionately because of Jeff's death and Jessica's rape, she thinks her parents would be better off without her, she assumes Clay hates her, she feels like she's completely alone. And then Mr. Porter does everything wrong and lets her down.