Daniel Boulton, 30, admits to the manslaughter of his former girlfriend Bethany Vincent and his son, Darren Henson, but denies their murder.
Ms Vincent and Darren, known to his family as DJ, were both found stabbed to death at their home in High Holme Road, Louth, shortly after 8pm on May 31.
It is alleged Boulton carried out the murders after driving 28 miles from his hostel in Alexandra Road, Skegness.
Boulton was prevented from seeing Ms Vincent by a restraining order but had continued to contact her and sent more than 900 messages over the weekend before the stabbings.
A Lincoln Crown Court jury heard Boulton would not testify during the trial and his murder defense relies on the testimony of a consultant forensic psychiatrist who conducted two interviews with him.
Defense psychiatrist Dr Pablo Vandenabeele said Boulton was questioned for four hours and described how his relationship with Ms Vincent began to break down in November 2020 when she continued to prioritize DJ.
Boulton told Dr Vandenabeele that DJ “did what he wanted” and was “abusing” him.
Dr Vandenabeele said Boulton recalled an assault on Ms Vincent which led to the restraining order, saying: ‘I threw her in the house’.
Boulton also recalled other incidents with Ms Vincent’s mother and DJ’s father where police were called.
Dr Vandenabeele said Boulton told him “I was in emotional turmoil” and started having suicidal thoughts as he tried to stick to the restraining order.
Boulton told Dr Vandenabeele he arrived at Ms Vincent’s home around noon on May 31, but she “kicked him out” and then continued to stare out the window.
The jury heard Boulton finally come in and told Dr Vandenabeele he had gotten on all fours and started begging Ms Vincent.
Boulton told Dr Vandenabeele that Ms Vincent knocked him down and laughed when he threatened to kill himself.
Dr Vandenabeele said Boulton told him he hit Ms Vincent when she tried to pass him, and then retrieved a knife from the kitchen.
Boulton told Dr Vandenabeele: “I guess I wanted to threaten her, I wanted Romeo and Juliet, I probably just wanted to kill myself.”
“It felt like I wasn’t there, the autopilot did the work for me,” Boulton added afterwards.
Boulton told Dr Vandenabeele he didn’t know how many times he had stabbed Ms Vincent, adding: ‘It was like she walked into the knife.
The jury heard Boulton tell Dr. Vandenabeele that he then went up to Darren’s room and inflicted the first injuries on his temple.
“The first stab wounds weren’t me,” Boulton told Dr Vandenabeele.
Boulton told the psychiatrist he left the property after smoking a cigarette.
Dr Vandenabeele told the jury he believed that at the time of the stabbings Boulton was in a state of emotional arousal that would have lasted for minutes.
“He (Boulton) seemed very agitated being ridiculed and attacked on his knees,” added Dr Vandenabeele.
Earlier in the day, defense and prosecution experts agree that Boulton suffered from dissocial personality disorder, but the prosecution argues that Boulton was still able to plan the murders carefully.
The jury must decide whether Boulton suffered from an abnormality caused by an admitted mental condition that significantly impaired his ability to control himself.
Dr Vandenabeele said the condition meant Boulton had a lower threshold for becoming violent and demonstrated a callous disinterest in others.
The jury heard there was a history of Boulton having had contact with mental health services at various times dating back to 2004.