Coroner Brigitte Windley is presiding over an inquest into Leon's death.
Detective Sergeant Chris Power, who was second-in-charge of Operation Lambeth, the investigation looking into Leon's death, said Roberts was a regular user of drugs, including heroin and cannabis.
He considered it "likely", given Roberts' habitual drug use, that he was affected by drug-taking when Leon was in his care the day he suffered his fatal injuries.
Text messages recovered from Roberts' phone were "absolutely littered with drug talk, drug use", Power said.
"There was extensive drug use by him," he said.
Power said Roberts talked about coming home and "lighting up" and smoking on the couch or in the kitchen.
Roberts had an "inherent predilection" towards anger, violence and bullying, Power said, which was triggered by drug use or when angered at being challenged.
While Power accepted there was considerable potential for the autistic Leon to injure himself, he believed that the boy's fatal injury was a culmination of physical assaults by Roberts on him.
Of Leon's 44 injuries identified in the post-mortem examination, police were not able to say if all the injuries the same day.
The second day of the inquest continues.
