21: Kimetsu No Yaiba Episode

When his parents discovered his monstrous nature, they attempted a murder-suicide to stop his sins. In a rage, Rui killed them, believing their bond was fake.

Rui was born with a frail body that left him unable to walk or go outside. Muzan Kibutsuji appeared and "saved" him by turning him into a demon, giving him a powerful body at the cost of consuming humans. Kimetsu no Yaiba Episode 21

The episode opens with the final moments of Rui, the Lower Rank Five demon. When his parents discovered his monstrous nature, they

Shinobu Kocho attempts to kill Nezuko, but Giyu Tomioka intervenes to protect them, violating the code he previously upheld. Rui killed them