A property agent heard a "voice of conscience" in his head telling him to teach his girlfriend's cheating ex-boyfriend "a lesson".
Wong Guoliang, 34, called the renovation contractor to work on his house, then donned a surgical face mask and hid a parang behind him - before carrying out a gruesome attack which left his victim hospitalised for 10 days.
Wong Guoliang, 34 - who suffers from a personality disorder - pleaded guilty to causing grievous hurt with a deadly weapon yesterday.
He left Mr Fredrick Toh Cheng Chye, 36, with a fractured skull and wounds to his arm, cheek, forehead and back. More than 20cm of Mr Toh's intestines also had to be removed as a result of the attack.
At a district court, Wong also admitted to three other counts: driving under disqualification, driving without insurance and driving without the vehicle owner's consent.
Six other charges will be taken into consideration in sentencing at a later date.
The court heard that around February last year, Wong found out his then-girlfriend, Ms Liao Sihan, had broken up with Mr Toh after finding out that he was married with a child and had been cheating on her.
Wong heard his "guardian angel" and "voice of conscience", named Michael, telling him to teach Mr Toh "a lesson" in case he tried to harm other women.
On May 28, using the name Michael, he called Mr Toh and feigned interest in hiring him for renovation work. They agreed to meet at 11am the next day at Hillview Warehouse in Hillview Terrace.
The next morning, Wong, who was banned from driving, took a friend's car and drove it there without his consent or insurance coverage. He put on a surgical face mask and hid a parang. His lawyer, Mr Steven Lam, said when Wong saw Mr Toh, he "recalled all the bad things that Mr Toh had done to Ms Liao and to other women".
Despite his injuries, Mr Toh managed to flee and ran to a nearby carpark, where he waited for help.
Deputy Public Prosecutor Kong Kuek Foo asked for four years' jail with nine strokes of the cane for the assault, saying the courts needed to send a clear signal against "such senseless acts of violence".
Mr Lam said his client was also traumatised by the attack and has difficulty sleeping. He asked for 2½ years' jail with six strokes of the cane for the charge, which carries a jail term of up to 15 years.
Wong is expected to be sentenced on May 12, pending a further medical report on Mr Toh's condition.