After stabbing Madezelle Smit, 28, Shaun Pretorius took her car and her dog and drove in the direction of Vereeniging, Captain Shado Mashobane said.
He stopped on the R59, got out the car with the dog, and ran in front of a moving truck.
Smit’s body was found with multiple stab wounds and she died on the scene.
Mashobane said it was believed that the two had broken up a week or two ago but did not know what triggered Pretorius’s attack on her.
Police have opened an inquest docket.