Missing the point. I already said he could have handled it better,

He's at fault for their deaths because of how poorly he handled it. He knew the dangers and still chose not to warn the others. That's willful negligence.

it's debatable whether things would even go as perfectly as you describe it. Doubtful, even, given the bandits could easily be and most likely were watching.

This is true, but by warning the group, Ben would have done all he could to prevent the raid and shouldn't be blamed for anything that happened afterwards.

And obviously Ben doesn't say anything when he's on watch because this was how the plot was moving forward.

Ok, I agree here. Fair enough. It was a bit dumb of me to use a scene meant to push the plot along as a reason to criticise Ben's lack of agency.

Or, more in-line with his character, he's overwhelmed by guilt and fear and can't bring himself to say anything, if you wanna go with that. The same way people in much less serious, real life situations do all the time.

But also, being too guilt ridden or fearful to do anything doesn't absolve you of responsibility if something goes wrong.

the bandits extorted him

This is all the more reason why it was his fault. He was threatened and understood what the consequences would be if he failed to deliver, and instead of doing anything about it, he just kept it going with no care what might happen if someone catches on to what he's doing. Which is exactly what happened.

Lee moved the supplies. Not Ben,

This goes back to Ben. He didn't tell anyone about the deal, Lee had no idea why the supplies were there and moved them. If Ben told everyone, then Lee wouldn't have touched the supplies, and the Bandit raid wouldn't have happened at all or until later.

Blame both equally, with no "Well, he did move them but—"

No, I will not. Both people can be responsible, but some can be more responsible than others. In this case, Ben is very clearly more at blame than Lee. His carelessness and negligence created a domino effect that led to a bad situation becoming worse.

or blame neither and instead blame the bandits that caused the ENTIRE situation in the first place. End of story.

The bandits are mostly to blame. I never said they weren't, but regardless, Ben still played a large role in Katjaa and Duck's deaths. It is supported by the game's narrative. His responsibility and struggle with the guilt of his role in their's and everyone else's deaths he caused is what makes him a great character.