Daniel Osborn, 24, from Longshore Drive, Shoreham, entered a takeaway in Brighton Road, Worthing, on 15 April 2017 and was verbally aggressive towards staff, police said.
A 54-year-old man, who was visiting the town, confronted Osborn – who followed him outside and punched him twice in the face, knocking him unconscious, according to police.
Osborn continued to kick the victim while he lay motionless on the floor causing multiple injuries including a fractured eye socket, before student Amir Aghaei pushed Osborn off the victim, said police.
Osborn threatened to kill him if he called the police, then hit the unconscious victim three more times before running away.
A woman cycling past stopped to administer first aid on the victim, while her partner Gilles DuBourg chased after Osborn, police said.
Osborn hid in an alleyway and ambushed Mr DuBourg, knocking him to the floor and kicking him repeatedly in the face causing multiple facial fractures, police confirmed.
Police arrested and interviewed Osborn on 8 May 2017.
He denied that he was the person caught on CCTV, claimed to know nothing about the second assault, and refused to answer any other questions but over a year later entered a guilty plea at court, said police.
At Hove Crown Court on Monday 15 October, Judge Shani Barnes gave Osborn a six year custodial sentence for assault occasioning grievous bodily harm and two years for assault occasioning actual bodily harm, to run concurrently, confirmed police.
Detective Constable Noel Simmonds said: “This was a needless incident that resulted in serious injuries to people simply trying to protect others.
“The sentence imposed not only reflects the severity of the harm that Osborn caused, but the need to protect the public from his violence.
“The case also demonstrated the best of Worthing: three strangers, each putting the needs of others above their personal safety.
“All of the victims and witnesses involved with this case showed great courage in seeing the case through.
“I would particularly like to thank Mr Aghaei and Mr DuBourg who stood up to Osborn despite seeing the violence he was capable of.”