In MGS when you're told you like killing its 100% ludonarratively consonant with Solid Snake and reflects the player's actions. In MGS2 when you're mocked for your lack of agency it's once again 100% consonant with Raiden and the player's relationship to the character and the game. Your argument suggests that MGSV is trying to say that all those actions mentioned previous are indicative of Venom, his bravado, and his villainy, yet the game never acknowledges them even one time.
I feel like this is an unfair comparison following your previous paragraph, as you're looking at the first two games much more broadly than you are V. I could argue that it's hard to imagine any number of the things you can do being the actions of the characters in those two games if we were to also look at them through a magnifying glass after all.
What I presume the person you're responding to is talking about in regards to V and certainly where its narrative and the player's actions line up in a similar way to your point about the first two games is that, to paraphrase the title of a great article on the game, you become entrenched within its war economy. You grow so desensitised to your expanding power to the point where you don't think twice about developing a nuke to protect yourself, you don't see yourself as a parasite stealing from these other countries and you don't think about what it means every time you "recruit" these people to your cause, which by the end of the game is simply perpetuating warfare around the globe.
Where this lines up with Venom Snake as a character is something you seem close to understanding about him, which is that he's numb to all this himself. Whilst I agree that he's stoic and somber, I have to disagree that he's merciful, which implies (as many people seem to argue is the case with him) that he's heroic, which isn't true. The best demonstration of this is when you bring the child soldiers back to base. It could be called a mercy that he didn't kill them like the job requires him to do but his plan for them, until Miller steps in with his own, is to add them to Outer Heaven like everyone else. The idea that they - Diamond Dogs, that is - would be able to help them move past their trauma turns out to be nothing more than one of several of the game's fantasies associated with them too.
Incidentally, on that last point, if you're looking for where the game acknowledges these things, Huey and
Paz are the ones to do so. Again, I don't think you were being fair to V here, because there is no direct analogue like there is with a character such as Liquid in MGS1 to tell you to your face that what you're doing is wrong. The whole point is that you're not necessarily supposed to see it that way. Instead you have these two characters and, of course, the irony is that one of them is a murderer who may or may not be guilty of other crimes but who in any case everyone connected to him in the game deeply dislikes, including Snake; and the other
is a figment of Snake's own imagination, his pain that he doesn't let show and presumably what he knows to be true.
I don't think any of this takes "heavy lifting" on the player's part. These are things talked about within the game, sometimes at great length, and I think the structure of the gameplay, which Death Stranding went on to take in another direction entirely, reflects this from beginning to end.