Evans said Hamilton told him he was unhappy in the domestic relationship.
Jurors were told the couple had financial problems, and Evans extended credit to the young parents.
"He'd cut his arm and was unable to work," Evans told the court.
"Because money became tighter they were ticking up a bit more."
"He had mentioned that he didn't particularly want Rikki to be ticking up any more so he'd asked me to put a stop to that."
Evans said he noticed a change in Simeon's behaviour.
"She had always been courteous and nice but within the last couple of weeks she had definitely started getting darker and her moods changed."
Evans told defence counsel Paul Borich QC he stopped extending credit to Simeon about one or two weeks before the homicide.
Borich asked if Simeon's mood change could have resulted from her credit being stopped, but Evans rejected that.
"Her attitude change came first, absolutely," the shopkeeper said.
Evans said he never saw the couple argue or be violent when he saw them in the shop or the wider neighbourhood.
Borich asked the shopkeeper if he ever went to the couple's flat near the superette. Evans said he never had.
Simeon was 18 at the time. The pair had a child together but the baby lived with a grandparent in Rotorua.
Jurors today also heard from military working dog handler Julian Young, who was on a ride-along with police the night Hamilton died.
The RNZAF leading aircraftman was with a police dog handler when the pair turned up to Simeon and Hamilton's Mount Eden flat.
The policeman went up the stairs and told Young to wait.
"He told me once he got to the top of the stairs that there's a knife there with blood on it."
Young said the police officer entered the flat through a window and performed basic first aid on Hamilton.
"Then he unlocked the door and told me to come up and guard the knife. I just remember him saying 'oh shit' and trying to deal with the situation," Young added.
Police officer Anita Sia was with Simeon after the 18-year-old was apprehended.
She told jurors Simeon seemed fine and made small talk before gesturing with her hands.
"She asked me about her hands. I asked her 'what?'. She put her hands up on the table with her palms facing up."
"I asked her if they're normally like that and she shook her head and said 'no'. I guess she just wanted to talk."
Jurors also heard from partygoers who were drinking with the young couple at a Mt Roskill house the evening before Hamilton was found dead.
One partygoer who consumed the bulk of an 18-bottle box of Steinlager beer said from what he could tell there was nothing amiss with Hamilton or Simeon that night.
Last week, a reveller who drank Cody's purchased from Scorpio Liquor on Dominion Rd that night said Hamilton had argued with another partygoer.
He said he encouraged Hamilton to "let it go" and have a drink, then Hamilton calmed down.
"Brendon was accusing [the other man] of touching Rikki or something."
The witness said the couple were not arguing when dropped off at their flat later that night.
The Crown has claimed Hamilton complained about Simeon being violent, angry and jealous.
But the defence has warned jurors against demonising Simeon for tensions in the relationship.
The trial before Justice Neil Campbell continues.
