Venom is pretty clearly more similar to the BB before GZ then BB himself in TPP. I really think that the best way to interpret Venom is someone who believes he is Big Boss, but lacks even facsimiles of significant portions of BB's actual memories and feelings. Part of the whole point of the BB and Zero split is that they have different interpretations of the Boss' will, so if two people could have such different feelings and interpretations of one person, how could Zero get Ocelot to truly replicate BB, and not just create a "robot" that possessed the general memories of him, without his true feelings on the events?
I've said if before, but I'd say Venom is really just "the medic" in personality and character, while being programmed to ignore missing memories and to basically just fool everyone and himself into believing he's Big Boss for as long as possible. The way he acts in the game is that of someone who doesn't really know why he's doing what he's doing, he has no great amount of agency and just believes what people tell him about "himself".
"The medic's" real personality and "self" sometimes shine through, and he is definitely a more forgiving and soft person than BB was after Snake Eater. What I'm saying, is he's still naive, and doesn't really do any dark actions without that action being heavily pressured and expected from others who he subconsciously knows to have more knowledge of what BB would do in any situation.
To paraphrase BB in 4 and the theme of 4, the Patriot's never succeeded in fully transforming one person to another, they didn't replicate Big Boss' "sense". They gave him as much of BB's memes as possible, but they couldn't truly impart BB's feelings and thoughts about the events he's experienced onto Venom, it's why he has no emotional reaction or hardly even a recollection of the AI pod. Venom, just like the player, has no true agency, since he's brainwashed to just "go with the flow", he wanders aimlessly as a person with no real true free will.
And Big Boss uses that, Venom, unlike Raiden has no hope for freedom. When Venom here's the truth, he's happy. What better person to know what "BB" should do, than BB himself. Instead of snapping him out of his hypnotism and making him pissed to learn he's a "fake", this just plays into his programming, and he rolls with it. If they truly replicated BB, he'd be absolutely furious about being played by Zero, and who he'd feel was the "real fake", the true BB. Instead he is happy to follow his programming, until of course, he finally realizes that he's screwed in the end.
So, basically I'm saying, Big Medic really isn't even a replication of Big Boss, his "sense" is completely different. So if he found out that the hospital was sacrificed on purpose, he'd be shocked. But if he learned soon after that Big Boss was the one who did it, he wouldn't object, since he is programmed to do whatever he's told Big Boss would do, even if he never really feels what Big Boss would feel.
Raiden at least wasn't literally programmed to lack free will. Poor Venom.