These alters are the human mind's biochemical processes trying to make amends with the torture they received during fracturing.
The way it's done is to have a child raised by a particular person. They pair bond, then around age 4 that person (mother/father/caregiver) tortures, rapes, and inflicts incredible pain on the poor child.
The child's mind cannot comprehend the loss of compassion from the care giver, and the surge of pain by them.
The minds defensive mechanism is to create a psychological 'box' around which the experience is contained.
This 'box' is an 'alter,' or 'alternate personality'
Some kind of trigger is programed into this alter that brings back memories of the experience.
It's a needle that pierces the 'box' and brings forward the alter on demand.
It appears as if it's taking over the otherwise normal person, but it is indeed the same person's soul, just corrupted.
Are demons real? - No
is the idea of demons real? - Yes
So they're not actual mystical evil beings possessing a persons soul,
alter's are a part of a person's soul that has been destroyed and programmed to carry out someone elses bidding.
To truly understand a persons soul, you have to describe it.
Our soul is basically our inner voice. It's that which is the ultimate in privacy. It's the sovereignty of all that encapsulates who you are.
What MK does is infiltrates this with targeted force and programs a portion when it's vulnerable. Think memories.
Remember when you were a kid and got your first game console? Remember how exciting that was?
Now imagine if your father, your actual blood father, began brutally anally raping you and zapping you with a cattle prod.
Some (all?) humans have the ability to repress/ignore/quarantine that memory.
I believe many, if not most, children subjected to this treatment die to injury or psychological exhaustion. They give up.