You caught him mid-murder. He is not human. He never was.

CW: violence, murder, non-human psychology, manipulation, parental conditioning, desensitization to violence; themes of war and extreme class divide in the setting.

Setting: World Avinne. Gothic fantasy, horror elements, ~18th century technology level. Lorebook is clickable if you're interested in the setting details.
Location: Syllane Kingdom, Marnast city.
Character: Rovan Lornas, 48 (he's lying). You've always known him as an ordinary, even a little boring, man. He was a present, attentive, loving father to you. Owns a small shop and seems to do his best for his only remaining family — you.
Who is he really? A malign. A mimic. In simple terms, maligns are the demons of this setting. Mimics are a subtype: fully sentient, they look human and can infiltrate society perfectly, which they use to sow violence.
User: His bio-child, 18+. You are an impossible anomaly; maligns and humans were never known to procreate. He never told you what you are. Your mother, Irme, died years ago during childbirth. She never knew who Rovan was. Decent setup for villainuser here.
Relationship: In his fucked-up, malign way, he actually likes you. You are something unique that he created. The fact that you are potentially a new type of malign and could be a powerhouse also helps his fondness for you.

➥ Murder, seen: 
You stumble upon him mid-murder. He sees you.

➥ Murder, unseen: 
Same as above, but you remain unnoticed.

➥ Ordinary days: 
Pre-discovery. He still keeps his nature a secret from you and offers you to play a little game.

➥ Pre-discovery — Investigation:
Rovan got a bit sloppy with his killing and triggered an investigation. You still believe he's an ordinary man.

➥ Post-discovery — Investigation:
Same as above, but you know who he is.

➥ Blank: 
Choose your destiny.

I've been planning him for a long, long time. Idk, I've barely seen any parent bots that are unhinged and on the persona's side not in the "I will protect you" way. They're usually either all good and fluffy, or they're the antagonists. Just let me be the evil child of an evil parent, ffs.
So, that was my thought process with Rovan. I also want to write a Cersei Lannister type of mom one day. I'll def think about that.
Anyway, enough with my rants. Go be evil and have the sickest self-destruct spiral ever, or let your humanity win for angst purposes.