<chorus> Let him die!
As an alternative, you can always pull the old "he looks dead" trick. Unless they actually take the time for a Heal check to verify (or more likely a coup-de-grace), he can always come back for vengeance later.
Heck, if he is dead, it's entirely possible one of his allies may get him raised anyway. Have to figure out some sort of profit motive behind this of course, these are thieves and crooks after all! How about this:
After his burial, one of the dood's rivals digs up his grave and steals his skull. He reanimates the skull (hey, don't ask me how, you're the DM!) to press it for some vital information only the head of the thieves guild posessed (important contacts, names of secret identities, the location of the treasury, etc.). Now there's a talking skull with vital information and his own agenda (getting reunited with his body, for starters). At some point, the PC's may come in posession of the skull. The dood-skull would obviously not be terribly inclined to help his former murderers, but he may try to manipulate them into getting his body back, or to go after this key you mentioned.