Billy Hickman told jurors the elder victim, Effingham County real estate developer Philip Heidt, confided in him about the affair and the feuding it caused between his sons. Hickman worked as a financial adviser for both slain men.
"He tried to get in the middle of it, like most parents would," Hickman said. "They threatened to kill each other, that's what he said."
Craig Heidt, 42, faces life in prison if convicted of murder in the August 2008 slayings of his father and brother, Carey Heidt. Prosecutors say the suspect killed both men with a shotgun while they slept at his parents' house in Springfield, northwest of Savannah.
He's also charged with shooting his mother in the face at close range. She survived the attack, but was unable to identify her attacker.
Defense attorney Dow Bonds says that Craig Heidt isn't the killer and that his only wrongdoing was sleeping with his brother's wife, Robin Rast Heidt. Bonds says police based their case on gossip and conjecture rather than physical evidence.
Hickman testified Philip Heidt was angry about the affair. He recalled one meeting during which the father said he wanted to remove Craig Heidt from his will, but Hickman said he talked him out it.
"He just could not stand the idea of his family being torn apart," Hickman said. "This marital affair was really driving him up the wall."
Prosecutors say Craig Heidt killed his father and brother to maintain the affair and in hopes of benefiting financially.
Hickman said Philip Heidt's net worth was about $15 million at the time of his death. He said Carey Heidt, who worked with his father in real estate, had a life insurance policy worth $3.5 million, which was put into a trust fund for his three children.
Monday was the fourth day of testimony in the trial, which could last the rest of this week.