So we can cross Giovanni and, obviously, Battler. Right now, it might be better to assume that these were two separate murders. I don't believe that Battler would kill Giovanni or vice versa.

[ they were good boys, brent! ]

So that leaves us with four people. The question is regarding the note that Battler had...though I don't believe there is any way to confirm if it's Battler's handwriting, now is it?