The trial, which began last Monday, had heard from Alison Walsh who was part of a group of people who Mr Mills was with in a bar in Ballynahinch.
She had stopped to allow the victim to get a takeaway before planning to drive him home.
When Mr Mills got out of her vehicle, a verbal altercation broke out between him and Foster, which quickly escalated into a physical fight.
CCTV footage showed both men pushing each other, before Mr Mills fell to the ground.
Foster punched him 12 or 13 times about the head, the court was told, before walking away.
The defendant had left the nearby Cloisters bar to get a taxi home just minutes before the incident happened.
Last week, the court heard from Mrs Walsh’s husband Anthony, who rushed to Dromore Street to help his lifelong friend.
Mr Walsh, a trained first aider, said that as he knelt down to help Mr Mills he heard a voice from behind saying “Let the b****** lie there, I’ve only broke his nose”.
He denied falsifying anything he had told police, and said the incident had left him in shock.
“What I witnessed was brutal to say the least,” he said.
The owner of the takeaway to which Mr Mills was going had pulled the shutters of his shop down just seconds before the attack took place.
He noticed both men in the area and said he had an “instinct there was going to be trouble”.
Meanwhile, whilst being taken into custody, Foster told police “it was self defence” and “he jumped me first”.
He was said to be “very upset” when told Mr Mills had died.
Brought back to his cell, Foster sat down on the bed, put his head in his hands and said words to the effect of “what have I done?” and “he started it”.
The court was told that a “scuffle” had broken out between the two men in a bar in Dundrum in 2011.
Although no punches were thrown, the court heard that they had their hands around each other’s throats.
The first three of eight interviews involved Foster, who had been in Belfast earlier in the day, answering “no comment” to police questions.
The interview transcripts were read to the court and jury.
When asked why he thought Mr Mills wanted to fight with him, Foster said: “Just the way he was talking to me, I just knew”.
He said Mr Mills’ demeanour was one of “anger, arrogance”.
He told officers Mr Mills was trying to egg him on to fight, but he didn’t want to.
“I just thought he had a bust nose, a bust mouth.”
He told police: “I should have just ran away”, that he wished he hadn’t went to Cloisters and he wished he had “just went home.”
Probation reports will be compiled before sentencing takes place on 24 January.