The Crown has argued Li was consumed with hatred after years of legal battles with Yang.
Jurors heard Li allegedly told a mental health support worker he wanted to "chop" a lawyer up and separately told an associate he wanted to butcher Yang with a meat cleaver.
But Li has rejected those claims.
And on Friday, he said it was wrong for the court to give much credence to what a "mentally crazy" person might have said.
Li also suggested nobody in their right mind would commit a blatant daylight killing in a location where escape was difficult and witnesses were present.
Jurors have been told the stabbing happened on a Monday morning in front of horrified onlookers.
"I think if you wanted to murder someone, you should at least do it at night-time, when there aren't people to see," Li told the court on Friday.
"Only stabbing once would cause death. There is no need to stab 12 times," Li added.
"To make sure the job was done, that's why you stabbed her 12 times," Webby said.
The court on Friday also heard from consulting psychiatrist Dr Graham Collins.
Collins said he examined Li and went to a police station with a mental health nurse to assess Li on July 29 last year, shortly after Yang was killed.
"He knew what was going on and where he was," the psychiatrist said.
Collins said mental health services in Auckland, Christchurch and Queenstown knew of Li and some of these providers had notes on Li from about 2005.
Jurors heard Li was on antidepressant Citalopram and had been prescribed Risperidone, which is an antipsychotic but is used for various other conditions, Collins said.
Collins said Li was diagnosed as having chronic depression.
He said it appeared Li had not slept well or correctly taken medication for a few days before the stabbing.
This could have impacted Li's cognitive functioning, Collins said.
But the psychiatrist added: "At my assessment, there wasn't any overt indication that having stopped his medication was having an impact on him."
Collins said clinical notes suggested Li was for a long time preoccupied with Yang and had rigid, obsessive ways of thinking.
"He had a long-term problem with anger."
Justice Pheroze Jagose began summing up the case to jurors shortly after noon on Friday.
"If it is not murder, then it's manslaughter," he told the jury.
Justice Jagose said for Li to be guilty of murder, the Crown had to first prove the stab wounds caused Yang's death.
Second, the Crown would have to prove Li meant to kill Yang or at least caused her fatal injuries without caring if she died or not.
The trial continues.
Domestic violence - do you need help?
If you're in danger now:
• Phone the police on 111 or ask neighbours of friends to ring for you.
• Run outside and head for where there are other people.
• Scream for help so that your neighbours can hear you.
• Take the children with you.
• Don't stop to get anything else.
• If you are being abused, remember it's not your fault. Violence is never okay
Where to go for help or more information:
• Shine, free national helpline 9am- 11pm every day - 0508 744 633 www.2shine.org.nz
• Women's Refuge: Free national crisis line operates 24/7 - 0800 refuge or 0800 733 843 www.womensrefuge.org.nz
• Shakti: Providing specialist cultural services for African, Asian and middle eastern women and their children. Crisis line 24/7 0800 742 584
• It's Not Ok: Information line 0800 456 450 www.areyouok.org.nz
