Palfreeman, from Sydney, was facing trial for murder and attempted murder after a violent brawl in the capital Sofia almost two years ago.

His father, Sydney pathologist Simon Palfreeman, said the family were shattered by the news but vowed to fight on.

The 23-year-old Palfreeman had gone to the aid of a gypsy being assaulted by a group of drunk football fans, Dr Palfreeman said.

"This is an act that can only be described as heroic," Dr Palfreeman said.

"While as a parent I would have preferred he stayed safe and out of harm's way, as a human being I am proud a young man tried to help despite such dangerous circumstances."

"The emotional appeal came as prosecutors asked for Palfreeman to be given a life sentence, and the alleged victim's family called the Australian a "professional killer".

On December 27 2007, the former St Ignatius' Riverview student was on a night out in Sofia when he became involved in a violent confrontation.

How the fight started is a key area of contention, with Palfreeman claiming he went to gypsies who were being assaulted by a gang of up to 25 drunk football fans.

Surrounded by the men, he says he was forced to produce a knife that he carried because of six previous incidents where people had pulled knives on him in Bulgaria.

University law student Andrei Monov, 20, the son of a prominent police psychologist, suffered a stab wound and died. Three others were also injured, including 19-year-old Anton Zahariev, who suffered multiple knife puncture wounds.

The group of local men have denied assaulting anyone and say it was Palfreeman that lashed out at them.

Monov's mother last night told the court: "It's totally abnormal for a young man of this age to go out in the downtown of a capital city with no ID, no mobile phone, but with a 30cm knife and a screwdriver.

"The blow he dealt to my son was that of a profession killer."

Palfreeman had just enlisted with the British Army and was on leave, helping a friend renovate a house in a town just outside of Sofia.