Damian Rzeszowski showed no emotion in the dock as the presiding judge, Sir Michael Birt, read out the verdicts that two jurats - similar to magistrates in the UK - had reached.
Throughout the trial, the defendant had constantly stared down at the floor, not looking up at any time.
No members of the victims' families were in court, but the public gallery was full of curious onlookers.
Judge Birt, who is the Bailiff of Jersey, said Rzeszowski would be sentenced on October 29.
More top news
The world's top scientists have called for a ban on autonomous weapons and "killer robots" that can attack targets without human control.
Growth figures published today are set to show the economy bouncing back after a weak start to the year.
A stagnating jobs market has created a blockage in the career progression and earning potential for many younger works, a new report says.