I know a lot of people in the Metal Gear community are sad and disappointed that we did not get to see Big Boss downfall in the game. I would take this into a different prospective and my interpretation of the game's ending. Instead of witnessing Big Boss downfall, you were a victim of it.

Think about it. The medic in MGSV probably admire Big Boss like the rest of his troops. He was basically their hero. When the medic sacrifice himself for Big Boss, that showed great dedication. Zero, in order to protect Big Boss, wanted to create a phantom or a decoy so Big Boss could survive. The medic was already expendable since that moment. Zero was essentially creating another Skullface. Taking away the medic face, identity, name, and origins away all for the sake of Big Boss.

He became Venom Snake. Since Venom is supposed to represent you, the player; you get to create your own avatar. What you are supposed to look like. You could spend a lot of time on it as you choose, but it doesn't matter. Your entire existence was erased. This is all messed up. This isn't right. And what did Big Boss and Ocelot do? They just went along with it. Ocelot even said that this is just your detour on your way to the hell, and just assumes that you would like playing the role as Big Boss for a while. They even said that they will use the patients and employees there as their shield to escape. What sort of hero does that? They knew that XOF was coming and they didn't say anything to the people there. The only reason why Big Boss saved you is because he needed you as a decoy.

This is all shady crap. And he did not tell Venom about this. Nobody told him about this until later. It's lies and deception. This is the exact reason why Big Boss left the USA and Zero. He left the USA because he was used, and left Zero because he did something without his consent. He's doing the exact same thing to you, the player. Through lies and deception, he allows you to believe that you are Big Boss, but you are a decoy. No one will ever remember who you are. No one will remember your name. Everything that you've done will just be credit to Big Boss and his legacy. Big Boss even thanks you for it, and tells you that you are doing a good job and together they are Big Boss. We all know this is a lie tho. Ocelot says that once the real Big Boss finish his true Outer Heaven, their allegiance will go to him and that there could be only one Big Boss. This heavily implies that he is going to try to kill Venom.

Kaz sees through this bull and vowed to take vengeance. Not only will he try to make Venom better than Big Boss, but his sons too so they can send him to hell.

Now usually loyalty will come to factor here. Venom should be loyal to Big Boss, so he should listen to his commander like a good soldier, right? Again think back why Big Boss leave America and Cipher? Through this exact same crap. This is similar to the events of Snake Eater. Big Boss was supposed to be loyal to his country, and he had to kill The Boss. However it was all covered with lies and deception. Instead of Venom killing The Boss, he kills himself in process of being a fake Big Boss. The government thanks Big Boss for killing The Boss. Big Boss thanks Venom for killing himself. This is hypocrisy. Big Boss is doing this as a means to an end, but he did something that he's initially against. He should be furious that Zero did this to HIS fellow soldiers like Zero did to him. Instead, he used you. He used the player. And Big Boss assumes that it would be a honor.

And Venom is different from Big Boss. Venom had the unfortunate burden to take on Big Boss memories. He felt sorry for them. He felt like a demon. He mentions it to Kaz. The problem is that his memories are NOT his own. He never did the stuff that Big Boss did, yet he is haunted by them. He isn't as hellbent on revenge like Kaz was, and was nicer. He tried to do the reasonable thing despite everything. If you see the nuke ending, Venom even interpretations The Boss will differently, striving for peace. Even his hallucination on Paz, it seems like he strives for Paz through her. The real Big Boss does NOT want peace. He wants constant war as well as defeating Cipher. He could be a lesser of two evils, but he does evil crap. Brainwashing kids in Metal Gear 2 to train to be soldiers in the future.

If anything, Kaz may have represent what would Big Boss would do at this time. Do you feel Big Boss would let Huey go despite his connections to Cipher after what they did to him?

And think about the comparison between Skullface and Venom. They are more alike than we thought. Both lost their face, their origins, name, and both became phantoms of Big Boss. While Skullface does cleanup work, Venom enhances Big Boss legacy. Both were going to be expendable. I think this is why you see the Venom turning Skullface in the trailers or Venom sees Skullface a couple of times after his death.

This all contribute to the ending of MGSV when Venom finds out the truth. Now this is my interpretation of the ending, and many people will have different conclusions after experiencing it. When Venom first sees his reflection, he still sees himself. Big Boss is giving a speech about how they can both shape the world as being Big Boss together and commenting about how he is his own man. Like predicted by Ocelot and Boss, you (the player) like the idea being Big Boss. Venom flips the tape to the Operation Intrude n313. Venom then looks at the mirror again. He no longer sees true self. He sees a fake. A phony. A constant reminder of what he is not. He is nothing but a decoy. His horn grew because he's done things that he is either ashamed of or anger has filled him. Regardless, he punched the mirror. He is angry at Big Boss. And possibly revenge has consumed him. Just like Kaz said at the end, he wants the phantom and his sons to become strong enough to kill Big Boss. This could be the thing that makes Venom snap. Being used by the person he admires the most.

tl;dr: Big Boss is an asshole and a hypocrite for letting Venom believes that he is him and he might try to kill you later on. And he let the patients and employees in the hospital die because they were "shields" for him to escape.