I think I posted this previously, but I'll say it again: Morgana Pendragon was not destined to be evil, she just had so much untreated head trauma combined with wanting to please Morgause that she kinda went batshit. Moral of the story, if you think you're possibly concussed, get it checked out.
Edit: Also, prolonged exposure to Uther would drive anyone insane, so that didn't help
This can happen! Traumatic brain injuries, especially repeated traumatic brain injuries, can greatly increase aggression and can increase the likelihood someone becomes a serial killer. She has all the background necessary, too. She was raised in an abusive home with an absent mother. Considering her father was Uther, it’s possible she’s carrying the low-activity Monoamine Oxidase-A (MAOA) gene, which is involved in regulating emotions and behavior. This gene is involved with coding the enzyme that breaks down serotonin, norepinephrine, etc. However, this gene is X-linked, so Morgana would have had to get it from her mother as well. If this is the case, Arthur likely did not inherit this gene, since fathers can’t pass it to their sons. Having the low-activity variant is linked with increased aggression and impulsivity and is commonly called the “serial killer” gene. And guess what? The low-activity variant is also commonly linked with having difficulty sleeping and night terrors! While we know her nightmares were from her being a Seer, it’s interesting that there’s a link there. So she has all the background and makings to become a killer, besides the whole “antisocial personality disorder” part.
