The Houston Police Department said the series of events happened on Saturday at about 1 a.m.
Commander Michael Collins said officers were headed to the scene when a man and his pregnant girlfriend were arguing at her apartment at 5600 Antoine.
The man allegedly kicked down her door and started to beat her up, according to investigators.
He left the apartment and made his way to the front of the apartment complex when the woman's male relative allegedly shot and killed him.
Officers said they heard the gunshots when they arrived, and the woman's boyfriend was pronounced dead at the scene.
The woman's relative was taken into custody without incident. No charges have been filed.
If you need help getting out of a domestic violence situation, call the Houston Area Women's Center 24/7 hotline at 713-528-2121 or call AVDA at 713-224-9911. You can also click here to chat with an advocate online. If you are deaf or hard of hearing and need help, call 713-528-3625.