I saw someone else talk about this and thought it was cool. I’ll do this in bullet-point form:
• Big Boss and Venom Snake initially continued working together from a distance after MGSV, but to help sell the deception of being BB’s body-double (in the event he’d need to assume that identity again, which he would in MG1,) he enlists Dr. Madnar to undergo an experimental surgery using robots and nanomachine injections to finally remove the shrapnel horn without causing a brain hemorrhage. By the power of “NANOMACHINES SON!”, it works.
Pictured below: Supposedly extracted from MGSV’s files: an aged Venom Snake without the horn. You can faintly see a scar where the horn would be.
• However, the nanomachines are experimental, with side-effects not being immediately clear. They end up damaging Venom Snake’s psyche and kinda make him go insane. He becomes ruthless and sadistic, embracing his reputation as a “demon.” He begins acting erratically, and not in line with Big Boss’ plans. This begins to sour their relationship.
• Operation Intrude N313 would be the climax of the wedge driven between Venom and BB. It would be BB sending Solid Snake to basically assassinate Venom so that then BB could take over Outer Heaven for himself. However, Solid “goes too far” by discovering the existence of Metal Gear and their plans to use it against the United States in order to take down Cipher/The Patriots. Venom does one last “F you” to BB by pretending to be him when confronting Solid, in order to destroy his reputation. With Outer Heaven and Metal Gear destroyed, the real Big Boss is forced to secretly start over in Zanzibar Land/Tselinoyarsk.
What do y’all think?