An Adelaide man accused of stealing a knife to murder someone he didn't know has denied he killed the victim.

A trial began in the Supreme Court of South Australia today after Shaun Falkiner pleaded not guilty to murdering 25-year-old Alexander Watts outside the Elizabeth Shopping Centre in April last year.

Prosecutors revealed the 28-year-old allegedly stole a 20-centimetre kitchen knife from a supermarket, which was used to inflict a single, fatal stab wound to the victim's stomach, during a violent altercation at a pedestrian crossing outside the centre.

Shaun Falkiner has pleaded not guilty to the stabbing murder of Alexander Watts (9news)

The court heard Mr Watts allegedly confronted Mr Falkiner, moments after he slapped a mutual friend.

The jury was told an "angry" Mr Falkiner hid the blade in his clothing and stuffed the packaging in his girlfriend's pram.

The court heard he was then encouraged by his girlfriend to attack the young father-of-three, telling him: "You are weak. You don't stick up for me. You're not a man."

A pathologist also told the court Mr Watts' death was “almost inevitable" despite witness' desperate efforts to save him.

Distraught family were in court as prosecutors showed the jury photos and CCTV of Mr Falkiner in the moments leading up to the stabbing.

The jury will travel to the shopping centre tomorrow, where prosecutors will recreate what they say unfolded in the final moments of Mr Watts' life.