I couldn't care less about "how dark" they go with Big Boss, I know they're going to show his fall into infamy, and I don't really care how they get there. There's no series of events he must do to get to where he gets to in Metal Gear in terms of infamy, at least not for me.
All I'm saying is that everything in the trailer HEAVILY points toward him killing those children POW's. From the interchange between Snake and the child taking the diamond, to their reaction, and the fact that both scenes take place in the same environment. Everyone else is saying this could all be editing magic (which I guess could be true) and/or that it doesn't fit Big Boss's character, which is a view I don't necessarily agree with but it's really all opinion so who cares.
I really don't even know what the sense of arguing about this is, since we'll likely know for sure come release.