In my articles on MGS4 I have emphasized the reduction of Solid Snake to a hired killer, stripped of the noble idealism we see at the end of Sons of Liberty. The “rapid aging” plot gimmick and the change of his name to “Old Snake” were early hints that his heroic leader status was going to be stripped away, and then we saw within the game itself that he doesn’t live up to his own ideals anymore; he gives up on teaching, and just focuses on trudging through his mission with little more fueling him than nanomachines and drug injections. People have tried to retroactively interpret his actions as something greater, but this shows a basic misunderstanding of the “Old Snake” character in my opinion.
Now Kojima has written an essay on the film Logan, and repeats the logic behind Snake’s transformation:
Back in 2008, I did something similar when I put an old Solid Snake into Metal Gear Solid 4: Guns of the Patriots. In the intro, Snake says “War has changed.” His codename was also changed to “Old Snake.” The letters “I” and “S” were removed from the word SOLID: “is” – or in other words “being” – had been taken away from him; he was now Old Snake. In much the same way, both Snake and Logan had their place in the world taken away from them. What place is there in the world for those whose very being has been wrested from them? All that remains is the end of their story – a story of their departure. I attempted to pull the same trick you see in Logan – of simultaneously writing an “end” for MGS4 while trying not to actually end the whole saga. [Emphasis mine]
This is confirmation. Solid Snake’s “being” is removed in MGS4, leaving only a shell of his former self. Once you remove who he “is”, he simply becomes “Old Snake”, the grumpy assassin trying to clean up the mess that is his legacy, devoid of ideals.