Spoilers for Metal Gear Solid follow.
For as sad as Snake's fate, presumably, really is he still had one of the best written character arcs in a video game.
With Metal Gear he was simply a soldier under the command of his Mentor, Big Boss, when he learned of his Mentors betrayal it then became his mission to Hunt down the one person he cared for most and when Metal Gear 2 came around he felt the obligation to end the Big Boss threat once and for all ending Big Boss's legacy, or so he thought. Learning that his one true friend, his mentor, was also his Father probably cut Snake like a knife. But he finished his mission anyways. Killed his father, his best friend, and his mentor in one kill. Later on after he retired he was called back not for the fact that it was his duty but because "he" was the enemy now. The "Snake" that had invaded Shadow Moses and demanded Big Boss's body under threat of nuclear war was, by the Governments accounts, Solid Snake himself so it was either he be hunted and shamed for "his" crimes or go to Shadow Moses to stop all out nuclear war. When he gets there not only does he find out that the enemy he's been hunting was his brother but that his nation, his home, had betrayed him and planned on blaming the nuclear threat on him no matter which way it went. While Metal Gear Solid 2 takes the approach that Snake has become a huge anti-nuke philanthropist. Creating Philanthropy and planning to end the Metal Gear threat once and for all. By all accounts he fails. Which sends him down a rabbit hole to fight his other brother, Solidus, and help his brother's adopted son, his adopted nephew; Raiden, his obligation to save the world fell on him as a personal matter. Not only was he to kill his family one by one but he was also to take responsibility for what Raiden would become. Metal Gear Solid 4 came around and Snake's last mission... was to die. Of course, he could have sat around and watched the Patriots take over free will and condemn the world to their tyranny but he felt it was his obligation to finish what he had started so long ago. To finish Ocelot once and for all. And he did. And in the end, he rid the Patriots, killed Zero, and watched his father give his peace and reasoning to Snake all for the sake of closure, to then have him die in his arms. His mentor, his first friend, died for the third time but for the first time in peace right in front of him. It was closure for Snake. It was closure for the series. Snake could finally rest in peace...
The sad irony of all this? He was just like his father. And Big Boss was just like his mentor, The Boss. Round and round in circles they went family killing family. Friend killing friend. As The Boss said "There's only room for one Boss and one Snake." And so Big Boss carried on that cycle. Till he realized the truth The Boss never knew. "There's only room for on Boss and one Snake. No, the world is better off without Snakes." And so Solid Snake's final mission was to die. And he completed his mission. Finally ridding the world of Snakes. Mission accomplished.
Thank you, Snake.
"This is good. Isn't it?"