You can beat yourself up over it, but the reality is that you're right: Some people handle the past differently from others.

You did the right thing by attempting to reunite them with their (presumably) priceless property. Most people likely wouldn't react this way. I know my parents lost a TON of personal items, including countless photos, when the moving company that was hired by the USAF to move them out of CA to another assignment went under. I'd imagine they'd both have been amazed, surprised, and incredibly grateful for someone to have gone through the trouble you did.

...but who knows? Perhaps there was a divorce or bad blood in that family. At least you can say for certain you have a clear conscience, though!