A jury has retired to consider whether a 17-year-old boy intended to inflict "really serious injury" on Melbourne teenager Laa Chol when he knifed her at a party.
The boy's state of mind at the time of the fatal stabbing is the key issue for jurors, who must decide whether to convict him of murder or the lesser charge of manslaughter.
Defence lawyer Sam Norton told the Supreme Court trial the attack was a "stupid, senseless act" but it ultimately fell short of the legal threshold for the crime of murder.
"It is tragic, unquestionably, but it is not a murder. It is a manslaughter," Mr Norton said in his closing address to the jury on Wednesday.
He argued the boy's actions were not those of an adult but of a teen who didn't pause to consider the consequences or think he could seriously harm or kill his victim.
Prosecutor Kristie Churchill said the boy didn't need to have premeditated the attack or have a specific motive to still be guilty of murder.
She noted he didn't strike Ms Chol in the leg or arm but stabbed with "at least moderate" force, penetrating her chest cavity and into her heart.
"An intention can be formed instantaneously and acted on spontaneously," Ms Churchill told the jury during her closing address.
"You can decide to do something and then act on it immediately.
"Murder can be entirely spur of the moment."
In the early hours of July 21, 2018, paramedics arrived at a 56th-floor city apartment to find Ms Chol, 19, "pulseless" on the floor and her clothes soaked in blood.
She had fought with a group of gatecrashers and was stabbed during a violent scuffle in the building's lift lobby after she suspected one of them had taken her phone.
Ms Chol stumbled back inside the apartment she had rented with friends for the weekend and collapsed.
The sac surrounding her heart and her right ventricle were pierced during the attack, causing internal and external blood loss.