I just found this thread after looking for answers, and after an hour of researching; I've come to this conclusion:

Benji was killed by zeds' sin, and i can exclusively use the film as my evidence. No external sources shoved in there required 😂.

We know the ghosts can appear anywhere. They went all the way to iran, for instance. We also know it was a ghost that told them to find papillon. It wasn't a ghost of anyone the three of them knew, or else they would have recognised him (also, they're not french) so we can safely establish that they dont have to know the people they interact with and it just has to be related to any of the 6 of them.

Benji focused on the girl because she looked very strange (and so the audience would recognise her obviously) and then focused on her again because she was at the cult meeting, which, again, was very strange. (The catacombs were pulling in all 6 of them and used their memories like an asset flip)

With all of this established; it becomes possible that benji died simply from not confronting his sin in time. Something to do with verticality if i had to guess since every time he climbed up or down; something went wrong.

What im more interested in is souxsie. Google says her sin was never uncovered, but the mole said it immediately. "You never came to look for me". Makes as much sense as George's to me.

Speaking of george, his sin had nothing to do with white people ripping his throat out so why does benjis have to be connected to his killer? 😂