Cache Ending?

How do we interpret the ending of Cache?

1. The video terrorizer wasn't Majid, the childhood peer, who killed himself in front of Georges?

2. The video terrorizer was actually Majid's son, Walid?

3. The video terrorizer was someone else, known only to Georges, who, for whatever reason, chose not to reveal, to keep "hidden."

4. None of the above, OR

5. The video terrorizer(s)was Walid, AND Georges and Anne's son, Pierrot?

The last theory comes to me via a detail about the film, revealed in the closing segment as the credits roll, by Roger Ebert. Ebert draws our attention to that closing camera shot, in front of the school, as people ascend and descend the stairs. In the upper left corner of the frame we see Walid and Pierrot talking to each other in a familiar, friendly way. They converse, and then part company amiably.

I didn't notice this during my first viewing of the film, and only caught it after reading Ebert's interpretation of the ending. BUT it does suggest that these two sons were friends - a detail never revealed, kept "hidden" in the film. So, it's plausible the two of them conspired to force their fathers to address their shared childhood trauma, or to use it to annihilate their fathers. Or, they simply engaged in a mind-fuck experience for no reason at all - for play - not realizing this awful history between their fathers (which, by the way is superficially disclosed, but, for the most part, also kept hidden).

I don't know.

jb