Daman Sehra, aged 23, admits the manslaughter of Amjad Khan – who he accepts he attacked at Yardley Close, in Oldbury, on June 8 last year – but denies murder.
His uncle Derek Brennan, 33, of Halesowen Road, Netherton, also denies both murder and manslaughter.
Sehra claims Mr Khan told his mother, Lisa Sehra, on a telephone call to take off her clothes in exchange for drugs, Wolverhampton Crown Court was told.
This led him to him "losing control and feeling a sense of unjustifiable wrong" before attacking Mr Khan, jurors heard.
But the prosecution claims Sehra actually attacked Mr Khan, who attended Lisa Sehra's flat after both defendants had arrived, because he was "jumped" by Mr Khan at an earlier date.
Judge Michael Chambers QC said: "The prosecution says Amjad Khan was attacked by both of them – not because of what he [Sehra] heard on a telephone call but because the defendant, Daman Sehra, recognised Amjad Khan as someone who had rushed him in Smethwick, or had been with the people who did."
He added: "Sehra admits the unlawful killing of Amjad Khan by kicking, punching and striking him with a bottle and toy – causing his death.
"But he denies wanting to cause him serious harm."
The court heard Lisa Sehra was "vulnerable" from her addictions to drugs and alcohol which made her son protective of her.
According to Sehra's defence, he went to the flat that night to "check up on his mother", having become "concerned others were taking advantage of her," said Mr Chambers.
At the flat, Sehra claims he heard Mr Khan speaking with his mother about supplying her drugs and "asking her to take her clothes off", jurors were told.
This was before Khan arrived at the property at 12.41am on June 8, with Sehra and Brennan attending the property at about 12.05am.
Lisa Sehra called 999 at 12.46am where it was "common ground that the assault on Amjad Khan was continuing", said Mr Chambers.
In relation to Brennan, the court heard he admitted grabbing Amjad Khan around the waist and punching him a few times.
"This was following Daman Sehra saying Khan had jumped him in Smethwick," said Mr Chambers.
Brennan also recalled Daman Sehra saying to his mother "how can you let him speak to you like that" after she had been on the phone, the court heard.
But he did not recall Lisa Sehra being asked to take her clothes off, jurors were told.
Both defendants have also been charged with attacking a 15-year-old boy at a different address in Oldbury earlier that night.
Sehra has also pleaded guilty to assault by actual bodily harm in relation to that incident but denies a further charge of attempting to cause grievous bodily harm with intent.
Brennan has denied both those offences.
The trial continues.