Ryan Esquierdo strangled Stuart Walker and then set fire to the 28-year-old's body at an industrial estate in Cumnock, East Ayrshire, last October.
Esquierdo subjected Mr Walker to "extreme and explosive violence" after the pair had a consensual sexual encounter.
The 19-year-old, who had faced a murder allegation, yesterday returned to the dock after earlier admitting to the culpable homicide of Mr Walker.
The plea was accepted after it was claimed Esquierdo was suffering from post-traumatic stress at the time due to being abused as a child.
Judge Rita Rae, QC, said Mr Walker had been the victim of "a brutal and senseless killing".
Esquierdo showed no emotion as he was led to the cells.
Mr Walker's heartbroken aunt Linda Woods, joined by a group of the barman's relatives and friends, said no sentence would heal the family's torment.
She also blasted the decision to accept the guilty plea to the reduced charge.
Mrs Walker said: "I don't know how it was not murder. He knew what he was doing.
"I don't know Esquierdo, but for someone to say that was his reason for what he did is shocking. He took this out on a person who would not hurt anyone.
"Stuart would have spoken to anyone – that was the way he was – and this is what happened. He was just at the wrong place, at the wrong time.
"Stuart just did not deserve this. He was one of the nicest guys you could meet. His loss has left such a big hole in the family."
The court had heard how Esquierdo and Mr Walker had been out separately with friends in Cumnock in October last year. They later met each other by chance as they made their way home in the early hours.
Esquierdo was lying on a wall sleeping when Mr Walker woke him. The pair then walked towards the town's Caponacre Industrial Estate appearing to be on friendly terms.
Prosecutor Andrew Brown, QC, said Mr Walker and Esquierdo spoke about the teenager's sexuality.
The court was told how Esquierdo previously had a number of girlfriends, but his sexuality had been "the subject of discussion by his friends".
Mr Brown added: "Stuart Walker was only sympathetic with [Esquierdo's] conflicted position. The accused described feeling safe talking to Mr Walker."
The pair then had a consensual sexual encounter. But Esquierdo became unhappy and started to panic.
The court heard claims Esquierdo was abused as a boy and the situation he found himself in with Mr Walker brought flashbacks.
Mr Brown said this triggered an uncontrollable rage within the teenager and Mr Walker was at the end of "extreme and explosive violence".
The advocate depute added: "The deceased would have had no warning or sense of what was going to happen."
Esquierdo bit, punched, kicked and stamped on Mr Walker. He then strangled his victim, for around four minutes, until he was dead.
Mr Brown told the court: "There is [CCTV] footage which may be the accused setting fire to his jacket which he placed on the deceased's body."
Esquierdo texted his friend Mary-Ann Dykes after the killing claiming he and a boy had "just got jumped".
Miss Dykes headed to the scene where she met Esquierdo, described as "a total wreck" by this time. She found Mr Walker's charred corpse and stamped out flames on a piece of fabric at his shoulder.
Esquierdo called police and again claimed to an officer there had been an attack by others and Mr Walker had been set on fire.
The teenager was examined and initially released, but was detained for the killing days later following investigations.
Esquierdo, of Cumnock, told police: "I'll go to my grave saying I did not murder that man."
The court was told that psychologists' reports concluded it was accepted Esquierdo was suffering from diminished responsibility at the time.
It was said post-traumatic stress disorder had sparked the brutal attack on Mr Walker.
Derek Ogg, QC, defending, said Esquierdo had "repeatedly expressed shame and remorse" for what he had done.
Judge Rae sentenced Esquierdo to 10 years and three months for the killing.
She added another year and nine months after he admitted attempting to defeat the ends of justice. He will also be supervised for five years on his release.
Judge Rae added: "It is patently obvious that you are capable of extreme violence.
"I cannot ignore too that, although having no memory of what you had just done, you had the presence of mind to attempt to cover up your crime and to destroy evidence.
"This included setting fire to Mr Walker's body and making up a story for the police that others had attacked you and that they had killed Mr Walker.
"You kept up that pretence for some considerable time."