In closing submissions, he said the attack against Ms Ross was "unrelenting" and there was evidence that she had been struck at least once while on the ground.
"Spines" of blood radiating from the victim's head where she was found proved that, he told the court.
With at least 14 blows to her head and 32 defensive wounds to her hands and arms, the killer's intent was clear, Mr Bates said.
The Crown's case was that blood found at the site suggested Ms Ross had been attacked as she was loading rubbish from her van into a skip.
Her blood in the van, which was found crashed into the skip, showed she had tried to escape, but Mr Bates pointed to a bloody gloved hand print on the vehicle's door as being significant.
He said that print was from the killer, who had dragged her out of the van and closed the door before continuing the frenzied attack.
Though the Crown did not have to prove a motive, Mr Bates suggested that, in this case, it was clear.
"[Merritt] felt unfairly treated by Karin Ross," he told the court.
Ms Ross had filed a formal complaint about the accused parking in a disabled spot and his behaviour; and Merritt had received a letter which said he may lose his job.
"She became the focus of his anger about the work situation," Mr Bates said.
He told the jury Merritt's reaction in the days after Ms Ross's death was also telling.
"He had no feeling, upset or remorse after the incident," he said.
Colleagues gave evidence of that, as did the defendant's parents, and in a police interview two days after the attack Merritt told them he did not like Ms Ross, the jury was told.
Mr Bates believed the person who killed Ms Ross would have known of her movements.
The shift was to be the victim's final one before she went on annual leave for some time and Mr Bates said Merritt would have known that.
"On the face of it, it would be the last time for a period this opportunity was going to present itself."
The Crown's case was based on circumstantial evidence, Mr Bates acknowledged, but he said that did not make it inherently weak.
He compared the prosecution evidence to strands of a rope which combined held significant weight.
"When you look at them together you'll be well past the point of being satisfied [of Merritt's guilt] beyond reasonable doubt," he said.
"I accept your task is a sad task, it's always an unpleasant task, but I suggest to you in this case it's not a difficult task."
Defence counsel Anne Stevens will give her closing address this afternoon.
