Perhaps we can trace it back to whether they died with regrets...or if the person in question's soul was corrupted by one of the seven deadly sins...these can both play a large role in how a soul behaves once it has left this mortal plane...Or maybe we can ask the all-knowing one regarding the topic of the dead...the Necronomicon...Book of the Dead. You heard of Demonbane? Maybe you should ask Al-Azif that question...she IS the living spirit of the Necronomicon, after all.