You're playing the Medic, a star soldier specializing in field surgery. He's the guy in the body armor with the gun, posing with BB in the picture the doctor shows you. BB says in the Doublethink tape: "He was always our best man." He's the guy who threw his body in front of BB to shield him from the bomb blast in GZ. Those are Paz's bones lodged in his body, the girl he failed to save. He wound up in a coma, and Zero conceived a plan to turn him into a body double: Punished Snake, a.k.a. Venom. He was already BB's best apprentice, so he had the experience, knowledge and skills. It was just a matter of making him think he's BB.
But it obviously didn't change his personality. Look at the difference between Ishmael (BB) and Ahab (Venom). Ishmael is all assertive, aggressive, Type A. Ishmael is a charming extrovert, wisecracking in the heat of battle ("I gave her a light, she took the short way down," "I'll run interference," etc). Note how Ocelot lights his cigar like a well-trained lapdog. Now look at Ahab, the soft-spoken, slow-to-anger imposter. He's primarily vengeful when acquiescing to the anger of his more assertive sub-commanders, but by nature he's more introverted, passive, Type B. He becomes more benevolent and emotionally vulnerable as the game goes on (the Paz tapes, sparing Huey, the angst over M43, etc).
In the end, the question is how he ultimately received the truth. The smirk and nod when he's told he's one-half of BB and they built the legend together suggests he's OK with the plan, in the short term. But notice how when he turns the tape, the Diamond Dogs logo in the background turns into the Outer Heaven logo. There's a time skip, to the end of MG1 when Solid Snake is at his doorstep. Venom is now covered in blood, his horn long. Solid Snake is about to kill him, and I suspect Venom is no longer happy to be the meat shield for the real BB. He punches the mirror, perhaps rejecting his fate. But the scene is open to interpretation, so that's just my take.
I feel bad for Venom. I think it's a neat way to show how the real BB turned. Instead of seeing a dramatic Darth Vader story where BB shoots up kids and commits other heinous acts, we see a story from the viewpoint of his victims. BB went along with a plan that robbed his best soldier of his identity and put him in harm's way. He went along with a plan that put -all- of the Diamond Dogs in harm's way. And he agreed to a plan where they would put the hospital in harm's way. We're seeing BB go against the values of the Boss because he is a hypocrite and a coward. He is now treating people like a means to an end, rather than the end itself.
