If a human or a creature of any kind could corrupt their soul to become a demon, if possible. How could they do it and how can another stop it or reverse it?
demons arent like demons u see in science fiction. That which many call demons, are just spirits with low vibrations. Negative actions lower a persons vibrations, positive ones rise them. By improving yourself, you can rise higher in the "spirit world" and not be a "demon" any more.
No one can become something else while they're currently living as human
Given if some people believe in reincarnation
Several literary works of art that were not, and never were, intended to be interpreted as magical facts or occultism informaiton, used demons and likewise spirits as a literary device known as symbolism, personification, allegories, and so on so forth. Or like doppelgangers.
Most likely I get what you're trying to say. To say that, you must believe Demons are "evil" and they always want to hurt humans, to kill them and such. As you said you think they're "unpure spirits" for that. And you possibly things Angels are "good", always willing to help humans and such. But trust me, that's not the case at all...
First of all, "evil" and "good" are concepts made of humans, to declarify a negative/possitive-to their eyes; action. There's no evil or good, their just words, everyone has the freedom to behave as they desire. As you understand, there is no "good" or "bad" person either.
So essentially, unpure soul or something are Just words. They don't mean anything..a Demon doesn't have "unpure" soul. Nor an Angel has a "pure" soul. Demons and Angels have just different soul energy types, but not different personas as they existed as humans.
If someone acts like a psycho on earth and wants to kill everyone etc, that's a mental issue. To be reversed or stoped, it needs treatment... If you mean though to become a normal Demon, well that's impossible. No human soul can become a Demon until their body dies, and no Demonic soul can exist into a human body...