You might want to put Spoiler in your title.
The guy who was on Trial, but he was found Not Guilty. He had the murder weapon hidden out in the tree house. The Lawyer was out celebrating with him after the trial and went to open the secret compartment to see if the old Marajuana was still there. The guy told him not to open it ...
I was pretty disappointed in the ending. I was hoping it was going to be the crazy neighbor or the Mom.