A murder trial jury was shown a video of the accused trying to save the victim after a pub attack in South Tyneside.
Newcastle Crown Court heard that father-of-two Ronnie Howard was punched twice and kicked once by Michael Wilson for "no readily apparent reason" during a night out at the Prince Edward Pub in South Shields on May 4.
Mr Howard, aged 31, who was described as an "excellent father", suffered a traumatic subarachnoid heammorrhage in the attack and could have died within "minutes, if not seconds".
He was pronounced in dead in hospital that night, despite the efforts by specialist medical staff.
Wilson, 35, of Hylton Avenue, South Shields, has admitted manslaughter but denies murder and is being tried by a jury.
Footage from inside the bar at the time of the attack was played in court and showed Mr Howard - who had been out with his girlfriend that day - crashing to the floor of the pub after the "full force" blows.
The CCTV coverage shows a brawl then breaks out in the bar while Mr Howard remains on the floor, not moving.
Wilson, along with others, is captured on film performing CPR and doing chest compressions to try and save his victim.
Prosecutor Mark Guiliani said Wilson and Mr Howard had been in the bar with their separate groups of friends when a "dispute" broke out.
Mr Guiliani said Wilson may have lashed out to stop Mr Howard, who was powerfully built, from intervening.
The prosecutor said: "No doubt, fearing Mr Howard would intervene on his friend's behalf, he decided to prevent Mr Howard from being able to.
"To do this, he intended to cause him really serious harm and knock him out.
"Why else use two, full force punches? Why else kick him, when all he was doing was standing close?
"Why else punch him so hard, without warning, not just punch him but to immediately follow up those two full force punches with a kick?
"The crown say it was to make sure once he was down he stayed down.
"The crown say you can be in no doubt, when Mr Wilson attacked Mr Howard he intended him really serious harm."
Mr Guiliani told the court Wilson's case is that jurors cannot be sure it was his assault that caused the traumatic injuries nor that he intended the cause Mr Howard really serious harm.
Mr Guiliani added: "That footage is powerful, compelling evidence that the defendant intended Mr Howard really serious harm."
A post mortem examination showed, among other injuries, Mr Howard had a heavy bruise around his jaw.
Mr Guiliani said experts say such an injury is "typical" of being the cause of further, internal damage that leads to traumatic subarrachnoid haemorrhage.
Wilson denies murder.
The trial continues.