I would say Louis is the murderer.Brian is informing Gus(as reference to the date (8(August)/6/2015)."..I could die on this very day, 8/6/2015. I am leaving this message in hopes to tell you .." this means Brian is informing about the other male character (..remember his name..) who is Louis.
Louis
Because:
Brian is informing Gus (as reference to the date (8(August)/6/2015). "...I could die on this very day, 8/6/2015. I am leaving this message in hopes to tell you ...".
This means Brian is informing about the other male character (remember his name...) who is Louis.