He obviously had stolen and pulled off selling the silver bars prior to any kidnapping or murder of Beverly or Greg. I wonder what/when he decided to go further and why? (the gold bar) and if so, if he admitted to murder, and admitted to theft of the silver, then why not the gold.....there is yet another piece missing to this puzzle. IMHO
We don't know if he admitted to the theft of the silver. They have proof it was him that sold them to the dealer - he used his real name on the receipt. They might also have him on video. On the other hand, the gold is still missing...