The 45-year-old had recently split up with his 21-year-old girlfriend but had tried to see her at her father's house in the village of Hofstätten an der Raab at 9pm on Thursday evening, according to a report from state broadcaster ORF.
She wasn't home and her father told the love sick man that she was still at work. The man then pulled out a handgun and searched every room of the house - finding the woman's new boyfriend, her seven-year-old daughter, and her uncle.
The boyfriend fled into the bathroom with the child, but the 45-year-old took the woman's father into the back garden and shot him at close range in the head - which was witnessed by the victim's brother.
The killer then went back to his car, sat in the driver's seat, and shot himself in the head. Police said that both men died instantly.
Neighbours who heard the shots called the police. Harald Winkler from Styrian state police said that the bodies of two males were found on the property and confirmed that one was the home owner.
Police are focussing their investigation on what happened in the days leading up to the murder. The surviving family members are receiving counselling.