This is the way I'm going to interpret the ending now. You get the tape from Big Boss he tells you you're Big Boss too. You smirk (the player) because ok that's kinda cool. Then after you listen to the intrude tape and the time lapse (in game and real life) happens Venom and the player are both pissed. Venom becomes because he's being used by the real big boss and then the player is mad this dumb twist just happened. That's why you become Demon Snake in that scene. So pissed that you played this whole side story for a shitty ending that you metaphorically have blood on your face and a huge horn. An ending like this could only come from the mind of Hideo Kojima, a true master.
Fuck this game's story.