Can someone explain the ending of Saw?

I just watched the movie Saw, and don't quite understand the ending. So Zeb the orderly was set up by the old man who was in the hospital bed?

So who was the killer throughout the preceeding incidnets? The old man was the killer the whole time? I don't believe someone of that age could have orchastrated all of those elaborate set-ups. For instance, the shotguns on the cieling attached to fishing wire that killed the cop? An old man couldn't have done that.

I know, it's just a movie, but this is bugging me now and my wife and I can't agree on who was really the killer the whole time.



Steve