In many stories, the transformation of human to demon has been interesting to think about. This of course extends to mythology: In multiple folklore, it's said that Lilith became a demon of her own will. In Japanese folklore female demons such as Kijo's and Hannya's transformed into demons out of hatred, or jealously, a curse, or a crime. With those 2 cases, they seemed to be born from the negative and/or destructive nature of humans.
So for this, let's say all demons are anatomically and physically close to humans, almost like a subspecies, and their genetics are compatible with them, which allows the latter to have the potential of transforming into one, not evolving in normal means (think of like shapeshifting, but it becomes permanent.) This means both already evolved to what they are.
I have two ways of a human transforming into a demon: One is by a powerful enough demon that can "corrupt" their genetic data, transforming them into a demon. The other is from a human's own negative nature, which if strong enough, can transform them into a demon. Which brings me to the question: How are these two ways of transformation possible?
EDIT: I'm not asking how can a demon evolve from a human. I'm asking how can a human who's already a human transform into a demon. I'm trying to see how could this be marked as a duplicate if I'm asking for something completely different. EDIT: I made it less vague so people won't make it for evolution.