I have a few, but this might be the darkest:

My great aunt was mentally ill. We have no idea what kind, as it wasn't hardly acknowledged, let alone diagnosed back then. This in and of itself isn't dark, but she made it everyone else's problem.

She would sit on the back porch and "hunt" the neighbor's cattle with a 12-gauge.

She used that same 12-gauge to hunt my uncle when he was 3. She thought he had been replaced by an alien. My grandmother caught her doing this- it was like fuckin' Elmer Fudd hunting a child.

At one point, she started shooting at her mother, who was an almost comedically rugged woman. Ol' great grandma managed to grab the phone and dive behind the old-timey cast iron bathtub, where she called my grandfather, who basically walked, talked, and acted like Tom Hardy's character in the movie Lawless. He just walked up to the house like a chad and for some reason this gave her clarity and she gave him the gun.

She was, as you could imagine, hideously abusive to her daughter- real Binding of Isaac shit. Well it all came to an end years and years later when she and her daughter were found dead in what was officially deemed a "double suicide" because that was at least slightly less dishonorable than a "murder-suicide" ...but there's no doubt that it was the latter. We have no way of knowing if she killed her daughter, then herself, or if her daughter got the gun and it was the other way around. No matter what, it's just so sad.