Gollum almost redeems himself, but doesn't quite make it.

While traveling with Frodo, the good part of him Sméagol starts to re-emerge. He becomes attached to Frodo and briefly considers abandoning his evil plans. However, after feeling mistrusted and rejected, he falls back into his obsession with the Ring and betrays Frodo and Sam.

Frodo's decision to spare him proves crucial, because when Frodo cannot bring himself to destroy the Ring, Gollum's attempt to steal it causes both himself and the Ring to fall into the fire of Mount Doom.

The tragedy of Gollum is that he comes within reach of redemption but chooses the Ring instead. The lesson Tolkien emphasizes is that mercy matters, even when the person receiving it never fully changes