The murderer is the guy who killed himself, in front of Jennifer, and he also followed Mason, Jesse and Dante. The murderer (his name was never mentioned, but I know it) is in love with Jennifer, and he saw both Mason and Jesse as threats to his relationship with her, therefore he killed them. That was his motive, pretty much.
I know it leaves a lot of questions, and I really don't mind. Glad you like it, though.