Minutes into this, while his pregnant wife was struggling and kicking her legs on the bed, Teo formed a plan to end the life of his daughter and kill himself in order to "reunite the family" in death.
Teo, 43, who is contesting two charges of murder of 39-year-old housewife Choong Pei Shan and his daughter, purportedly told this to a psychiatrist at the Institute of Mental Health (IMH).
Teo said that at first, he strangled his wife because he wanted to "shut (her) up", according to a report by Dr Derrick Yeo that was revealed in court on Thursday (Jul 4).
He was angry over an insult Madam Choong had aimed at him.
According to Dr Yeo's psychiatric report, Teo claimed Mdm Choong said: "(Daughter), you look, your father really useless. So useless I shouldn't have married him."
He added that he responded by saying: "I really hate you talking about me in front of our daughter. I told you many times not to do that."
Convinced he had to shut his wife up to end her nagging, Teo fetched a towel from the bathroom and looped it around Mdm Choong's neck, strangling her.
Mdm Choong, who was sitting on the bed, did not scream, but used both hands to grasp and pull at the fabric constricting her neck, the court heard.
Teo purportedly told Dr Yeo that he noticed his wife's legs kicking and moving on the bed as she tried to resist him.
He said he told his wife in Mandarin: "Shan, you got to let go already, I owe a lot of money. If you struggle, you will suffer because I owe outside a lot of money. You will suffer."
Teo had racked up S$70,000 in debt from credit card bills, school fees and gambling.
After his wife slumped on the bed, Teo called out to her to see if she was dead, said Dr Yeo in his report. He then strangled her a second time with his hands as he saw bubbles at the ends of her mouth and thought that she could still be breathing.
READ: Murdered four-year-old adored by everyone in my family, says mother of accused in Woodlands double deaths
After this, Teo asked his daughter to sit on his lap. He told her: "Don't be scared, Papa (is) here."
He then looped the towel around his daughter's neck, and told her in Mandarin: "Go find your Mummy first, Papa will come soon."
He said the girl cried softly, then loudly, before making unintelligible sounds, as he strangled her.
After noticing bubbles at her mouth, he removed the towel and strangled her with his hands, saying in Mandarin: "Darling, go find Mummy already."
ACCUSED NOT OF UNSOUND MIND: PSYCHIATRIST
Dr Yeo was giving testimony for the prosecution on the third day of the trial. Prompted by Deputy Public Prosecutor Han Ming Kuang, Dr Yeo said Teo's account showed that he had been capable of planning and thus was not of unsound mind at the time of the offence.
"His behaviour after (the offence) showed that he was in full cognitive and volitive control of his actions," said Dr Yeo.
There were no delusions or hallucinations, nor was there any abnormality of mind, said the consultant at IMH's department of forensic psychiatry.
Dissecting Teo's behaviour before, during and after the alleged offences, Dr Yeo said Teo was angry at the insult from his wife, but was "goal-oriented" at the time.
This can be seen in the way he left the bedroom to retrieve the towel from the bathroom, said Dr Yeo.
His decision to "give up" on the family's lives together appeared "quite rational", said Dr Yeo, as Teo had told him he wanted to kill the whole family because he did not want people to come after them.
"The accused felt that due to his debts, his wife would be hounded by the people he owed money to, and he kind of made a unilateral decision as the father and head of the household to end all their lives together," said Dr Yeo. "This appeared to ... demonstrate some form of planning and goal-directed behaviour as well."
ACCUSED ABLE TO MAKE OBJECTIVE CHOICES: PSYCHIATRIST
The decision to kill his wife a few minutes into strangling her showed that he was able to make objective choices and consider alternatives to his behaviour, and that the killing was not something out of his control, said Dr Yeo.
The words Teo spoke to his wife and daughter were attempts to justify his actions, that he would join them in death, and that it was not wise to continue living with creditors hounding the family.
The intent to kill her came only a few minutes into the act of strangulation on Jan 20, 2017, in the master bedroom of the Woodlands flat they shared.
Although Teo said he wanted to kill himself, he was unsuccessful in his attempts.
Dr Yeo said: "The accused also told me that he tried various methods to try to end his life, but that he was unsuccessful. He apparently wanted to die on his own terms but many of the attempts he tried were either too painful, too difficult or not successful for him."
Dr Yeo explained to the court that a forensic psychiatric evaluation differs from a clinical psychiatric evaluation in that the overriding duty of the former is to the court.
"A high index of skepticism would be necessary in such a forensic clinical investigation," said Dr Yeo. In particular, such an evaluation would consider exaggeration of symptoms, and seeks not just to establish whether or not the accused needs treatment, but to ascertain the presence or absence of a mental disorder to guide the court.
CGH PSYCHIATRIST DIAGNOSED ACCUSED WITH DEPRESSION WITH HOMICIDAL INTENT
Earlier on Thursday, a doctor from Changi General Hospital's psychiatric team, which reviewed Teo a day after his arrest, took the stand and gave a differing view.
Dr Melonie Sriranganathan, a member of the medical team caring for Teo at the time, told the court that he had an alleged overdose of paracetamol.
Liver function tests conducted on him were consistent with a paracetamol overdose and he was treated for it, said Dr Melonie.
A psychiatrist who reviewed Teo gave a diagnosis that he suffered from depression with homicidal intent and persistent suicidal intent, the court heard.
Additionally, he was noted to have low oxygen saturation and may have inhaled substances which could potentially cause an infection or pneumonia.
Teo had told police that he tried overdosing on paracetamol as part of various suicide attempts following the deaths of his wife and child.
He also set fire to the bodies and slept next to them.
According to other medical notes from the hospital, Teo said he "had no regrets" and insisted he would die, as he had been told by the police that he would be punished with death for murder.
His only regret was that he had to take the life of his innocent young daughter, the court heard.
The trial resumes on Friday.
If found guilty of murder, Teo faces the death penalty.