When Ross Konidaris broke through the back door of his grandparents' Melbourne home and gunned down the couple at point-blank range, it was the culmination of months of paranoid delusions.
He had been found with a knife down his pants while preparing to search for a bomb under his house. He had told family members there were people hunting him.
Victorian Supreme Court Justice Terry Forrest on Thursday found Konidaris not guilty of murdering the couple by way of mental impairment.
Konidaris, a heavy drug user, was suffering from paranoid schizophrenia and was plagued by delusions at the time of the killings, Justice Forrest said.
"The accused clearly did not know the wrongfulness of his act," he said.
"His reasoning behind his actions seems based on paranoid fantasies."
Konidaris, 25, had visited his grandparents - Triantafillio Konidaris, 81, and his wife of 55 years, Stavroula Konidaris, 84 - at their Yarraville home on December 21, 2012.
He returned to their home just after midnight to wait with his shotgun in their garden shed as he built up the courage to kill.
Within a few hours, he burst into the house and shot the couple dead at point-blank range.
After his arrest, he told police he believed his grandfather once killed a man, and people would seek revenge.
He felt he had to kill his grandfather because their shared surname endangered him.
Justice Forrest said three psychiatrists diagnosed Konidaris as suffering from paranoid schizophrenia, meaning he was unfit to stand trial.
In the months before the killings, family members reported strange behaviour from Konidaris, including him repeatedly searching beneath his home for explosives.
He once found a piece of coal and believed it was the bomb he had been looking for, Justice Forrest said.
His father also reported finding him in the kitchen late one night with a knife concealed in his pants, and he spoke of needing a gun to protect himself from the people out to get him.
Konidaris, of Sunshine West, pleaded not guilty to two counts of murder on the grounds of mental impairment.
After finding him not guilty, Justice Forrest ordered Konidaris be held at Port Phillip Prison while a report on his psychiatric state was completed.
Justice Forrest will then decide where Konidaris will be held and for how long.