For the first time prosecutors have revealed why Roger Kingsley Dean set fire to the Quakers Hill nursing home: to destroy evidence that he had stolen about 230 tablets prescribed to the residents.

Dean, 37, an aged-care nurse at the home, pleaded guilty to all charges of murdering 11 residents in the fire in November 2011, bringing tears of relief from his victims' relatives.

His admission in the NSW Supreme Court on Monday allowed the Crown case to be released. It alleged that the night before the fire, Dean repeatedly went into the room where the home kept its addictive medications and stole 237 tablets of the narcotic painkiller Endone and one capsule of the morphine drug Kapanol. These can be bought, with a prescription, for about $85.

According to the Crown statement, to which Dean is yet to formally agree, he became extremely fearful when nursing home managers discovered the stolen drugs and called police.