Hamilton in 2018 admitted hitting Simeon but claimed he'd done so because Simeon had refused to let him leave the house.
Lummis said bank records showed the couple in May 2019 had only $28 between them.
Jurors heard the couple let Hamilton's mother in Rotorua look after the baby, who was about to turn 1 year old.
On May 17 that year, Hamilton and Simeon went to a friend's birthday party in Mt Roskill.
Lummis said Hamilton had appeared drunk but witnesses reported Simeon seeming withdrawn and morose.
The court heard Simeon told one partygoer she was upset about her daughter.
At one point, a friend lent Simeon a jacket.
"It was his attempt to get the jacket back later in the evening that led to the grim discovery of his friend's body lying lifeless on the kitchen floor," Lummis said.
The friend had no cellphone with him and tried waking neighbours, then flagging down cars to get help.
The court heard he reached a Mobil petrol station at about 2.21am on May 19 and emergency services were alerted.
The Crown said Simeon was seen in a blurry CCTV image running past shops at 2.29am.
Lummis said Hamilton was stabbed in the neck with a 20cm knife.
"It pierced downwards and even pierced the top of his lung."
There was no suggestion the killing was planned.
"For reasons we may never know, Miss Simeon got so angry with Mr Hamilton that she picked up the largest knife in the kitchen."
"She lashed out. And she lashed out in anger. She wanted to hurt him because she was feeling hurt."
Lummis said Simeon's account of events later oscillated, sometimes blaming Hamilton for the stabbing, another time asking for forgiveness.
The Crown did not accept self-defence motivated the stabbing and no cuts were found on Simeon's or Hamilton's hands to suggest struggles over a knife, Lummis said.
"There are no winners in a trial like this," she added.
"It's a tragic loss of a young life. It's a shockingly young age to be facing a charge of murder."
Defence counsel Paul Borich QC said it was unknown why an argument broke out that night in May 2019, and unknown what Simeon's state of mind was.
"Rikki-Lee Simeon and Brendon Hamilton probably should have not been together," he said in opening remarks.
"They weren't good for each other. They had their ups and downs."
He said jurors would need to know what happened in the Dominion Rd apartment before even contemplating whether Simeon had murderous intent.
"Be careful of falling into the trap where ... Rikki-Lee Simeon is blamed or demonised for collective failings in the relationship."
The trial before Justice Neil Campbell continues.
- Additional reporting: Miriam Harris
