This theory is probably going to make absolutely no sense and I’m probably gonna get hated for it. But I’m doing it anyway! Alright this is my theory...what if Seth killed Bryce? Before you go raging in the comments, here me out.
What if Seth somehow knew that Justin was friends with Bryce and wanted to get back at him for what Justin did to him. So he killed Bryce knowing that Justin would want to be drawn more into the truth. And maybe he wants him to dig deeper to get in trouble with the police and have his friends be punished for what he did. And maybe just maybe he wants him to go to a specific spot where Bryce was so he can finish Justin off.
I know this sounds ridiculous but that is one of my choices for who killed Bryce. What do you think?