She had not expected to see Bush at the house that night when he drove up accompanied by his new girlfriend, Maria Korotaeva, a 20-year-old who met the businessman while studying in Bristol.
Kukucova told the court that on seeing her in the house, Bush had told Korotaeva to go outside and the former couple had begun to argue.
She said that Bush had gripped her by the arms and neck and threatened to kill her and her family before going to lock the door to the house and returning to point a gun at her.
Korotaeva, who is due to be a key witness in the trial, told reporters shortly after the incident that the couple had found Kukucova in the house dressed in pyjamas.
Bush took her outside to wait in the car and had gone back into his house when she heard three shots and saw Kukucova run out, lock the door and drive off in one of the dead man's cars. Kukucova drove to Slovakia before giving herself up to police.
The public prosecutor in Malaga is asking for a sentence of 20 years for Kukucova, while private lawyers representing Bush's family have asked for 27 years if she is found guilty.
The trial continues.