Some people believe in dead spirits among all major religions of the world. I am not sure how true this belief is, but I do know that my girlfriend sometimes talks about how she has seen her dead grandfather on a few occasions, sometimes even along with other witnesses from her own family.
My questions are:
1. Are "dead spirits" sentient beings like humans, animals, hungry ghosts, Devas etc?
2. Why do some people not get a new life in a new realm and form according to their Karmic conditions immediately after death and instead remain dead spirits?
3. Do Devas, Asuras, Animals etc also become dead spirits after their death?
I have heard that the reason some people get stuck as "dead spirits" is because of their untimely death or premature death even when their Karmic life was still remaining in their present life. Like those who were murdered horribly, who ended up in accidents, or who died during wars and battles. This makes sense and doesn't seem to contradict the theory of rebirth in Buddhism. Is my understanding correct?
Namo Amitabha Buddha