One of the stab wounds pierced his heart and proved fatal.
Watkins said when she and another police officer arrived at the entrance to the tavern carpark, a 17-year-old male approached her with both hands up. He had blood all over him.
She said the teen complied when told to get down on his knees and he looked upset, scared and was shaking.
He said "it wasn't my fault" more than once, Watkins told the jury.
After the boy had been handcuffed and led into a police car, he said a man he didn't know had been stabbed and was lying on the ground.
Watkins said a bloodied Willems was dead by that time and blood patches were quite dark in some parts of his body.
Crown solicitor Mike Smith told the jury earlier that day, Willems picked up the accused and three of his teenage relatives to go on a drive around the mid-north in his new vehicle.
Smith said it appeared they stopped twice and bought alcohol.
CCTV footage in Kawakawa at 10pm showed everyone out of the car and violent and verbal aggression between different people before Willems' car continued towards Ōpua.
In his short opening address to the jury, Mansfield said the accused was concerned Willems was behaving inappropriately towards his cousins in the car and thought he needed to protect them.
The accused was under the influence of alcohol and cannabis supplied to him by Willems when the stabbing took place, Mansfield said.
"The defence position will be quite clearly he acted in defence of himself and in defence of his cousins, given the circumstances which occurred that night.
"The evidence will reveal, I suggest, that the young people in the car were very scared and frightened. He responded in a way he thought at the time was necessary to protect himself and his cousins. He wasn't intending to kill."
Mansfield said the 14-year-old was carrying a pocket knife like a lot of country kids did.
