A 130-kilogram teenager who allegedly stomped a diminutive 64-year-old woman to death in her western Sydney driveway dealt the fatal blows because "God" told him to, court documents suggest.

Imanueli Jone Degei - who would later allegedly tell police he was on a "bad high" at the time - had been discharged from a mental health facility two weeks earlier.

Police at the Carramar unit block where Kristina Kalnic died. Credit:Seven News

Court documents detail how Zivan Gajic watched from the driveway of the Carramar unit he shared with flatmate Kristina Kalnic as then 18-year-old Degei staggered down the street in an apparent stupor on August 25.

Degei appeared to have passed out in a flower bed when Mr Gajic called Ms Kalnic to come and see if the young man was OK about 1pm.