The accused Claremont serial killer had no idea his first wife was having secret weekend sex with their boarder but when he found out about the affair, he allegedly threatened to murder his love rival.
Bradley Robert Edwards, 50, denies murdering Sarah Spiers, 18, Jane Rimmer, 23, and Ciara Glennon, 27, in the mid-1990s during his marriage breakdown after his first wife had an affair, resulting in a child.
The man, whose identity is suppressed, testified in the Western Australia Supreme Court on Monday via video link from overseas and recounted a phone conversation he had with the former Telstra technician.
"He accused me of having an affair with (her) and I said to him, 'I thought that was plain and clear to see'," the man said.
"He said 'oh, I'll kill you', and I said, 'Well you know where I live, you've got my address'."
Contradicting evidence from Edwards' first wife, the man said they had sex before he became their boarder and the affair continued after he moved in.
"She would sneak into my room while Bradley was still asleep," he said.
"I thought she was playing a dangerous game."
She testified that one day, Edwards saw her hugging and kissing the man on the cheek, but he barely reacted and the boarder remained.
But the man said they kissed on the lips after looking through photographs together, including one from when she won a Miss Wet T-shirt competition.
Believing he had "overstepped the boundaries", the man began packing up his things, telling Edwards' wife: "I don't fancy sleeping here another night knowing what he's got in his room."
Prosecutor Carmel Barbagallo prevented him from elaborating.
The man said both he and Edwards cried over the incident and Edwards told him to stay, although he eventually did move out.
Under cross-examination, the man agreed Edwards treated him courteously and did not know about the sexual relationship while he was living at the house.
The court previously heard Edwards and his first wife separated between late 1995 and early 1996.
But the man's evidence contradicted some key dates prosecutors are relying on to show Edwards' "emotional upset" coincided with the murders.
Defence counsel Paul Yovich repeatedly poked holes in the man's memory, including where he was living on some important dates.