It has been stated said before that Metal Gear Solid V will show the transition from ‘Snake’ to ‘Big Boss’, but also the psychological reasons for this change. The character will experience ‘serious psychological damage’, according to Kojima, speaking to the French press. It will be a story of revenge that is quite cruel. The reason the game has such a dark tone is because Kojima wants to tell the story of this transition as effectively as possible.
In another interview (with 4Gamer), Kojima compares Big Boss’ situation to the one of Walter White in Breaking Bad. He is someone who kills and breaks the law, but you can understand his reasons. For those around him, he seems like a nice guy, but from another perspective you see a monster. If Solid Snake would have been in the same situation, maybe he would have chosen to do what is right. Big Boss did not, however.
To a certain extent, Kojima wants to convey Snake’s emotions through the other characters that surround him as well. One example of this is the final scene in Ground Zeroes. Kaz is expressing his anger more explicitly than Snake. But Snake too has every reason to be angry.
Either way, it’s clear that The Phantom Pain will tell the story of Big Boss’s ‘downfall’, and the beginnings of Outer Heaven as we know it from the previous games.