He said to convict Wiremu of murder, the jury needed to be sure that:
* Wiremu kicked Nia in the head on the Friday evening the Crown alleged.
* That if he had kicked her, he intended to injure her.
* That he appreciated death was a likely consequence of his actions.
* And not withstanding that, he took the risk anyway.
Mr Horsley argued no medical evidence existed to show that kicking a 3-year-old in the head would kill her, and pointed to evidence from a paediatrician who said Nia would probably have survived had she been taken to hospital immediately.
He said other assaults on the toddler in the days leading up to the Friday could have caused the subdural haematoma, or bleeding in her head, that killed her, and the Crown had failed to exclude this possibility.
Nia had been spun on a clothesline until she fell off, placed in a clothes dryer and wrestled, and witnesses spoke of her displaying symptoms consistent with a head injury in that time.
"We know that by 7pm on Friday, this young girl had suffered numerous, not insignificant, bangs to the head."
In terms of the clothesline incident, Mr Horsley said it had begun as a game and Wiremu had been only a spectator. "The ringleader in that scenario was Michael Curtis."
Mr Horsley said Wiremu was intimidated by his brother, easily led, and a slow learner.
"He may have been 17 years old at the time but he was operating at the level of a 12- or 13-year-old."
Mr Horsley also questioned the reliability of a child witness who spoke of seeing Nia being kicked in the head by the Curtis brothers, saying the child had given some evidence that was clearly wrong. That included saying Nia was hot and had a blood nose when taken out of the dryer, because tests had shown it would spin only two rotations carrying a child her weight.
Michael Curtis' lawyer, Jonathan Temm, also argued the child's evidence was unreliable, and said the fatal kicks never happened.
He said the child's evidence was unsupported by anyone else, and there was no bruising on Nia's forehead.
He also argued a series of events led to a gradual decline in Nia's condition, and said wrestling moves performed on the 3-year-old, though "sheer stupidity", lacked criminal intent.
Lawyers for the three other accused will give closing addresses today.