Ryan Esquierdo strangled Stuart Walker and then set fire to the 28-year-old’s body at an industrial estate in Cumnock, 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 admitting the culpable homicide of Mr Walker.
But the dead man’s family criticised the decision to allow his killer to avoid a murder charge.
The reduced charge 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 handcuffed to the cells.
Mr Walker’s aunt Linda Woods – joined by a group of the barman’s relatives and friends – said no sentence would heal the family’s torment.
“I don’t know how it was not murder,” she said “He knew what he was doing.
“I don’t know Esquierdo, but for someone to say that was his reason [being abused as a child] for what he did is shocking. He took this out on a person who would not hurt anyone.
“Stuart just did not deserve this – he was one of the nicest guys you could meet. His loss has left a big hole in the family.”
The court previously heard how Esquierdo and Mr Walker had been out separately with friends in Cumnock on 21 October last year.
They later met each other by chance as they made their way home in the early hours.
Prosecutor Andrew Brown QC said openly gay Mr Walker and Esquierdo spoke about the teenager’s sexuality.
The court was told how Esquierdo previously had a number of girlfriends, but that 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 got intimate, which was consensual. But Esquierdo became unhappy and started to panic.
The court heard claims Esquierdo was abused as a boy and that the situation he found himself in with Mr Walker brought on “flashbacks”.
Mr Brown said this triggered an “uncontrollable rage” within the teenager and that 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.”
Nine-stone Esquierdo bit, punched, kicked and stamped on Mr Walker who was of far greater build.
He then strangled his victim – for around four minutes – until he was dead.
Judge Rae told Esquierdo: “You have pleaded guilty to a brutal and senseless killing of a decent young man who appears to have attempted to show you only kindness and sympathy.
“For that, he lost his life in a most horrific way.”