Which begs the question, though I'm sure it has been asked: Who the Hell is Skull-Face?! Obviously he knows Big Boss in some personal-manner... I wonder if he's a former student of The Boss? Ground Zeroes loading screens specifically mentioned that Big Boss was the LAST student of The Boss...
Could Skull-Face be another student of The Boss we weren't allowed to know about? Could this overbearing theme of REVENGE include him? That is, could Skull-Face know and hate Big Boss for killing The Boss and being her "best" student?
The cassette tape in question just infuriates me, if only because it makes plain to listeners that Skull-Face isn't some Lone Ranger-mask wearing freak... He has history with Big Boss and (probably) The Boss. What if he was the first attempt at avenging Her and the world she wanted?
That is, Skull-Face is a former student of The Boss, learns she's been killed, goes forward with an idea to overthrow CIPHER (i.e.; Status-Quo). He doesn't really succeed, since he's focused on the person, not the stage (figuratively speaking)...
So Big Boss finally reaches a breaking-point, and obviously he goes with Skull-Face, since their goals are similar (to preserve the thought and philosophy of The Boss). But there's obviously a problem here, since Skull-Face must hate the man who took-down The Boss. But he plays along since, well, "Why not?" There's nothing to gain going against Big Boss and the Diamond Dogs... And plus, Skull-Face, assuming he was close with The Boss, would know that Operation: Snake Eater was a ruse for the East, designed to keep the West in-control of the vast fortune of the Philosopher's Legacy.