Re: So why was the old man killed?
I like the ledger idea posted by a lot of the people here, but I also agree that the WWII jewels thing might be a bit predictable given the guy's age. That does, however, I agree, also make it an awesome red herring.
I like the cape and cowl idea, I thought of something a little more non-descript. Well, at least, less out of place in a jewelry store.
If you don't mind going a little afield in sci-fi, say that one of the jewels that passed through the shop wasn't a jewel at all, but some sort of experimental power source. The break-ins are from some covert, secret government agency, trying to get their hands on it. The old man was killed accidentally when he rushed for the silent alarm during the first break-in, and the ledger holds the name of the person to which the jewel, which an ordinary jeweler with failing eyesight did not recognize as extraordinary, has passed.
EDIT: I just realized that I didn't elucidate a means of the power source getting mixed up in his jewels: it could have been put there for safe keeping from somebody inside the organization that "lost" it, so that they could eventually sell it on the black market, or use it to their otherwise nefarious ends. Or, in the complete opposite direction, it could have been taken to prevent something nefarious.