The 'occupant' of the tomb went missing many years ago, and once hope was given up of their return, the crypt was built as a way of saying goodbye for the community. This could have been a local hero, person of importance, a mage, etc. That being said, the 'loot' in the crypt consists of funeral offerings and grave goods donated by the same said local community. When these things show up in the would be heroes hands, the locals are going to be a special kind of pissed off. The main lesson to be learned is that not every crypt or tomb is a place to be looted. Some are places to show some respect for the dead. May the PCs be so lucky that upon their demise the friends they have made and people they have influenced would be willing to raise a memorial in their honor
I like the mystery. It gets the wheels turning.
Perhaps the corpse went the way of Yoda or "Ascended" and is now trying to look out for its descendants. Being non-corporeal, it is having a devil of a time. What luck for it that the adventurers caught its attention. Now it will follow them around, trying to get them to listen (unintentionally causing havoc in the process) until they agree to help sort out some injustice for its distant progeny.
I am also quite partial to the mystery of this submission. My addition is that when I think of a crypt it is usuall one for an entire family. Imagine a crypt intended for several generations completely devoid of bodies. Perhaps the family had some sort of special link to the underworld that made them particularly desierable as undead. Perhaps the crypt is so ancient absolutely everything has been eaten away by time. New Orleans buries their dead above ground to prevent the corpses from floating about during floods, perhaps something similar (or the reverse) happened.