I have a feeling that it could have been Olivia (Hannah's mom) who killed Bryce. I mean, first he rapes her daughter, which drives her to suicide, then he gets away with it relatively scot-free, despite it being clear that he did sexually assault Hannah. Olivia would be really pissed at him and for her, the only way for justice to be properly served is to take the law into her own hands and kill him.