I accidentally killed him and now i have no way of getting the house key of the building Mejula. Anyone know of another way to get it?
After a little time has passed, a tombstone will appear where you killed Cale. If you interact with the tombstone, you can pay souls to speak with Cale's spirit, which will allow you to obtain the key.
This same technique works for all (or almost all?) NPCs in the game. If the NPC in question had items for sale, you're also able to browse their wares this way.