Lukasz Czapla, denies murdering his son Julius by shooting him with a BB (ball bearing) gun, repeatedly stabbing him with a skewer and suffocating him with a pillow on 20 or 21 November 2020.
Patrycja Szcześniak feared her former partner had killed their son after receiving a text from him.
She found Julius dead at the flat.
Mr Czapla, a former IT technician at Fife College in Dunfermline, offered to plead guilty to a lesser charge of culpable homicide but this was not accepted by the Crown. He has lodged a special defence of diminished responsibility.
The jury at the High Court in Edinburgh heard in agreed evidence that on the evening of 20 November, or the morning of the following day, Mr Czapla, 41, killed his son at the flat in Muirhouse.
The court heard that Miss Szcześniak had earlier dropped her son off at the flat and said he was excited about getting pizza.
She said Mr Czapla seemed “fine, normal”.
Julius was happy to go with his dad as was Lukasz to have him,” she told police in a statement that was read to jurors by advocate depute Alan Cameron. Miss Szcześniak was 32 at the time of her statement.
The following morning she saw a text that had earlier been sent by Mr Czapla. She could not remember the exact wording but “that he meant he would kill himself and Julius”.
The court heard that she had not responded to earlier messages Mr Czapla sent, asking about her sex life.
She went straight to the flat and, through a letterbox, saw her ex-partner covered in blood and staggering about.
Miss Szczesniak said she started shouting and kicking the door.
She told police that when she got into the property Czapla indicated that Julius was in a bedroom where she found him on a bed.
“His head was covered in blood and his face was blue. I knew he was dead,” she said.
Miss Szczesniak said she got out of the flat and shouted to a neighbour to call the police, saying: “He has killed Julius.”
The hairdresser said she had met Czapla as he was a neighbour in the Muirhouse area of Edinburgh. They had a relationship for eight years and lived with their son.
She said Czapla was not happy to learn she was seeing another man and told her he had been prescribed with tablets for depression.
She told detectives: “I have never had any concerns as to the safety of Julius when he is with his dad.”
A post-mortem examination found that Julius had three steel ball bearings in underlying tissue of his head which had caused bleeding and a fractured skull.
There were also wounds to the boy’s left temple caused by the discharge of a BB gun.
He had also suffered a stab wound to the chest and pathologists found signs that were consistent with suffocation.
Agreed evidence revealed that during police searches at the flat, mushrooms containing Psilocin were found along with LSD, ecstasy and cannabis.
Mr Czapla was also said to have been stopped for drink and drug driving, speeding and crashing his car.
He has pleaded not guilty to all charges.
The trial, before Lord Beckett, continues.
