Crime & computers,? deleting ,double deleting?

I am watching Tru TV (Court TV) all of his plans to murder his wife and his affair are being used in court. He double Deleted, they even know the dates for everything. How can the person be so stupid? I have no clue on how all of this works, but I've seen enough TV to know you can't get away with a computer or a cell phone used in a crime!