Stuart Kelly subjected to "catastrophic" ordeal before his death, parents say.

Warning: This post discusses suicide and may be triggering for some readers.

The parents of Stuart Kelly – the younger brother of one-punch victim Thomas Kelly who suicided after starting university in 2016 – believe their son was the victim of “catastrophic” treatment that likely contributed to his death.

“He was one of the really popular kids [at high school], and he went off to university at Sydney, for one night at a college, and he came home a different person the following day. You know, it just changed him, he was broken,” Stuart’s mother, Kathy Kelly told Channel 9’s 60 Minutes on Sunday.

“We hadn’t seen him cry since Thomas died," she continued, "so to see he was just sobbing, uncontrollably, and he came home and he went into his room and he basically didn’t come out for the next couple of months, so you can only assume that something catastrophic happened to him that made him feel the way he did.”

Stuart Kelly was 14 years old when his older brother, Thomas Kelly, was killed in a one-punch attack in Sydney's Kings Cross in 2012. Following his brother's death, Stuart went on to campaign heavily against alcohol-fuelled violence and to support the work of the Thomas Kelly Foundation.

The Kelly family's campaigning, coupled with the death of one-punch attack victim Daniel Christie in 2013, eventually led to the controversial lockout laws being introduced by the NSW government the following year.

The public campaigning saw Stuart's public profile rise and sadly, often made him the target of bullying.

"There were death threats and things like that to our family," Kathy said, asking, "Who does that do to an 18-year-old?"

Attending the University of Sydney, Stuart was enrolled at St Paul's College (a campus with a number of complaints currently against it) but stayed just one night before returning home.

According to his parents, once home, Stuart didn't leave his bedroom for months and never told them the full story of what happened. Both Kathy and Stuart's father, Ralph Kelly, believe their son may have been the target of physical and/or psychological abuse by other students within St Paul's.

"[Campaigning] took its toll in the end ... We paid the price very badly for it," Ralph said.

"With Thomas, he was happy when he died. He was holding a little girl's hand and looking forward to the night ahead and how he was going to kiss her for the first time. Hopefully, he didn't see anything happen or anything coming, "Kathy said, "but Stuart was in pain and we couldn't see that and we couldn't help him."

Ralph added, "I think the pain is internalised and it will never go."

St Paul's College told 60 Minutes they would not comment while police investigations are underway.

Readers seeking support and information about suicide prevention can contact Lifeline on 13 11 14.