Remember the Psychic Link that both Sonic and Hyper Metal Sonic developed on the final battle, allowing them to feel what the other feels, and think what the other thinks because they are essentially the same? There's no implication that said psychic link never ceased, meaning that Sonic felt not only the dismembering Metal suffered, but also himself dying by lava the same way Metal did, and considering how Metal Sonic started to become more and more like Sonic, it means that his decision to die is something the real Sonic would have chosen as well if he was on his position, Sonic wasn't just trying to save Metal, he was desperately trying to save himself as well to no avail.
