He probably killed his second wife as well

On June 9th, 53 AD, Nero wed Claudia Octavia, his own stepsister. Nero nevertheless engaged in extramarital relationships, first with a freedwoman named "Claudia Acte" and then with "Poppaea Sabina," the daughter of Roman Senator and future Emperor Otho. Nero divorced his wife and forbade her from entering Rome when she began an affair with him. Twelve days after the divorce was finalized in 62 A.D., he wed Poppaea, who was carrying their first child, despite his claim that she was barren.

Poppaea's marriage did not turn out well. Nero allegedly kicked her so hard in the stomach that she died while they were fighting about something silly when she was pregnant with their second child. This is presumably how all married couples fight from time to time. Although the precise moment of her death and the cause are unknown, many contemporary historians also left open the possibility that Poppaea passed away while giving birth.