|Damian Rzeszowski killed six people including his wife and two young children|
Damian Rzeszowski, 32, stabbed Izabela Rzeszowska, 30, Kinga, five, and Kacper, two, in St Helier on 14 August last year.
He also killed his father-in-law, his wife's friend and her daughter.
After five hours of legal arguments earlier, the Jurat's took two hours to consider their verdict.
Earlier in Jersey's Royal Court, Rzeszowski was sentenced a 30 year determinate sentence in a UK prison.
The Judge, Sir Michael Birt, said: "The horror and brutality of these killings is hard to believe.
"Within a quarter of an hour or so six people had their lives cruelly cut short at the hands of this defendant."
Earlier in court, the prosecution had called for a 30-year sentence for Rzeszowski.
He had been cleared of murder but found guilty of manslaughter due to diminished responsibility.
As well as his wife and children, Rzeszowski also killed Marek Garstka, 56, Marta De La Haye, 34, and Julia De La Haye, aged five after a summer barbecue at his St Helier home.
During the trial in August, the court heard that Rzeszowski's marriage had been under strain after his wife confessed to a two-month affair.
It also heard he had threatened to commit suicide and had been suffering moderate to severe depression, which had caused an onset of psychotic symptoms.
Rzeszowski and Izabela Rzeszowska had moved to Jersey from Poland in 2004 and built a new life on the island.
They married there in 2005 and had two children, Kinga and Kacper.
|Kacper & Kinga|
|Marta & Julia|