As a teenager, any number of things can cause mood changes, including depression, stress, mental illness, trauma, etc.
Many people with or without these previously mentioned events/issues can and do draw dark things.
YOu say she blacks out and becomes a different person. How exactly?
Has she gone to a doctor for these events? If not, the likely culprit most of the time is mental illness. Possession is very rare.
You say murderous is one of these personality traits. How so? If she was murderous, that implies violent and possibly homicidal. Can you cite examples of these events?
Only after ruling mental illness, could you bother with considering possession.
Does she have an aversion for founts of faith and superstition, or items, trinkets, substances that people strongly believe thwarts/harms/scares evil spirits of any culture?