Why did Song Ruilai kill her mother?

Song Ruilai wanted to kill her mother because her mother's last wish was euthanasia. According to relevant information, Song Ruilai's mother's last wish is euthanasia. As a nurse, Song Ruilai euthanized his mother, which led to Song Ruilai's mental disorder. The Resolution to Break Up tells the story that Zhang Haijun (Hae-il Park), a criminal policeman who investigates a murder case in a mountainous area, meets Song Ruilai (Tang Wei), the wife of the deceased. Riley showed no signs of excitement about her husband's death, and his behavior was completely different from that of his sad relatives. The police think Riley is a suspect.