Wesley Dates, now 20, and his older brother Peter Dates, 22, are on trial in the NSW Supreme Court over the fatal stabbing on the evening of October 1, 2023.
The brothers had been drinking and watching rugby league with friends at an apartment in Miller, in western Sydney, when George Woods, 18, and Joshua Ratu, 19, arrived.
Peter Dates and Mr Ratu got into a punch-up on the street during which they each landed a number of blows before Wesley Dates became involved, the court was told.
It is not disputed Wesley Dates fatally stabbed Mr Woods before using a knife to seriously wound Mr Ratu.
He pleaded guilty to wounding Mr Ratu with the intention of causing him grievous bodily harm and to the manslaughter of Mr Woods, but prosecutors rejected the second plea.
Instead, they argue Wesley Dates is guilty of murder because he intended to kill the 18-year-old or at least cause him grievous bodily harm when he wielded the knife.
If they are unsuccessful in establishing intent, prosecutors will rely on manslaughter as a backup offence.
"Whatever the verdict you return in this case, Wesley Dates is to be held accountable for his actions on that night," his lawyer Tom Quilter told the jury during his opening remarks.
"Neither of those stabbings was an accident."
He said the now-20-year-old had accepted that what he did was wrong and criminal, but he acted rashly in "excessive self-defence" which didn't amount to murder.
Although Wesley Dates stabbed Mr Ratu, Peter Dates is accused of participating by forming an unspoken agreement with his brother to wound the other man with the intent of causing grievous bodily harm.
Either that or the brothers had a less specific agreement to inflict unlawful violence on Mr Ratu when they assaulted him together, prosecutors contended.
There was no agreement to wound Mr Ratu and Peter Dates didn't consider his brother would act in that way, his lawyer Denis Stewart told the court.
He said the then-20-year-old was so intoxicated that his vision was blurry and he was laser-focused on his one-on-one fight with Mr Ratu.
"He wasn't in a position to notice very much else that was going on around him," Mr Stewart said.
Peter Dates didn't see his brother stab Mr Woods and hadn't contemplated a knife would be produced to stab Mr Ratu suddenly and "without warning", his lawyer said.
The trial will explore the intentions and beliefs of the brothers at the time of the stabbing, the court was told.
It will continue before Justice Peter Hamill on Thursday.
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