A man who murdered four American university students has been sentenced to life in jail after avoiding the death penalty with one of the victims mothers disappointed the "loser" won't be executed.

Bryan Kohberger, 30, had initially denied the killings, however, he later pleaded guilty as part of a deal that ensured he avoided the death penalty.

Kohberger was accused of sneaking into a rented home near the University of Idaho campus and murdering Ethan Chapin, Xana Kernodle, Madison Mogen and Kaylee Goncalves.

The motive behind the former criminal justice student's murders has never been revealed.

It is also unclear why two roommates, who were at home at the time, were spared.

Post-mortem examinations showed that the four victims were stabbed several times and were likely asleep when they were attacked. Some sustained defensive wounds

Kohberger was arrested at his parents' Pennsylvania home weeks following the kills after a nationwide search.

The court heard victim impact statements from the four students' families in the presence of Kohberger's mother, Maryann.

Goncalves' mother Kristi revealed that she was disappointed that Kohberger won't be executed by firing squad.

"You will always be remembered as a loser, an absolute failure," she said.