Soul is soul no matter of devta's, human's, rakshasa's or anybody's.
But the soul is given the category as per soul's nature.
Even the worst person can be the best no matter he is rakshasa or not.
For Example, Prahlad was rakshasa by birth!! But he is the best devotee of Lord Vishnu whom all rakshasa hate.
At any point of time nature can change. But it is through "AWARENESS".
The person not aware of what he is doing is considered as "Rakshasa".
Soul is always a part of GOD. It is like shallow copy.
Each soul has basic properties of GOD like peace,love,bliss.
Even the angriest person will be calm and peaceful for more % of the time per day.
Some times some souls behaves like demon because they are given such role and only they can fulfill them.
But through the light of awareness they can change their nature.
Body dies, soul never dies.
Time change, thoughts never change.
But choice of thoughts may change anytime.
There is nothing like Hinduism, or no religion can make a person deity or demon, it is just the atmosphere provided to one in childhood to grow nature that results in small actions of his life.
I mentioned thought never die.
Good thoughts are deities and bad thoughts are demons. Do they ever die?
A taste of one particle of soul is taste of the whole sea.
The experience of self's soul is not different from the experience of supreme soul(GOD).
Why to stick with Hinduism?? Why only hinduism scriptures define what is demon?
Why can't we have our own definitions?
Whatever written some years ago was suitable to that IQ level of that time's people.
If they write "There was a demon with 1000 hands and some god killed him" --> Is this acceptable in this age of science???
Maybe some buffalo will think GOD is super buffalo may be with 4 horns!!!
Imagination has no end.... but how you relate it with reality is important.
Our choice represents our soul. Nobody is eternal except GOD.
Lord Krishna said
Oh Parth, you were not there or I or these all kings and soldiers were not there, it is not true, and you, or I or theses all kings and soldiers will not exist in future, it is also not like that.
As per Hindu philosophy everybody who takes birth has to die.Only Lord shiva has not taken birth, so only he is eternal.
Yes demons have to pass through birth-death cycles. Yes their souls are also a part of god.
For Example Ravana. Kumbhakarana
By soul, both were door keepers of Lord Vishnu's abode!!
Their names were Jay and Vijaya. Both were cursed to born in Rakshasa yoni for 3 births.
3 demon births
First Birth: Hiranyakh and Hiranyakashyapu
Second Birth: Ravana and Kumbhkarna
Third Birth: Shishupala and Dantavakra
In all of these three births they were cursed to be killed by Lord Vishnu as Varaha , Narmsinha, Rama and Krishna avatars of Lord Vishnu.
So they borne, died and their soul freed from curse.
And then they went to again as door keeprs to Lord Vishnu's abode after death.
reference : Jaya-Vijaya
Edits are invited for this answer