Of all the mistakes Sauron made, letting Gollum go alive was probably his WORST, well, other than him agreeing to join Melkor and not seeking pardon after Melkor's fall. It was a HUGE risk to let Gollum go free after him being interrogated. What if someone was able to figure out where he'd been and get the information that Sauron was after the Ring? If Gollum HAD been killed, the Nazgul could have been in the Shire before Gandalf and Frodo had enough information to figure out that Bilbo's Ring was the One Ring and that they needed to get out of the Shire and go and try and destroy the Ring. The only thing I can think of is that Sauron thought that maybe he could get Gollum to cause mayhem and that if anyone on the good side did encounter Gollum, they'd just kill him on the spot before they could get anything out of him. If Gandalf letting Gollum get away nearly proved to be the West's ruin, Sauron letting Gollum go free DID become Mordor's ruin.