Garry Kane, 41, wept at the High Court in Dumbarton as he told the jury he found Kathleen Millward, 87, lying in the kitchen and phoned for help.
He denied stealing money from her or that they had a strained relationship.
He is on trial accused of murdering the pensioner at her home in Stonehouse, Lanark-shire, on January 3.
Kane told the court he left her home around 6pm on the day she died to buy drugs. His QC Gordon Jackson asked where he got the money from and he said he had money from gran for Christmas.
The jury was told that while out Kane's mother Kathleen Kane phoned him to say she could not get a hold of Mrs Mill-ward on the phone, and asked him to check on her. Kane said he went to another man's house first to do with another drug deal before he went to see his gran.
Mr Jackson asked his client: "Were you partic-ularly worried at that stage?" The accused said: "Not overly." He added that a couple of times before when Mrs Millward had not answered her phone it was because she was in a neighbour's house.
Kane sobbed as he told how he later found his gran. He said: "I put a small lamp on and that's when I could see my gran lying on the ground in the kitchen."
Mr Jackson asked: "Did she look to you at that stage as if she was dead?" The former para-trooper replied: "Yes."
He told the court he phoned his mum then emergency services.
The court was told earlier by one of the men, David McFarlane, who supplied Kane drugs that Kane made a comment the day after Mrs Millward's death about hitting her.
Kane denied making the comment, saying he hadn't seen McFarlane since his gran died.
The Crown allege Kane killed his gran before going to buy drugs that day.
Advocate depute Jennifer Bain said: "You knew she was dead or dying before you left the house, you did not call 999, you called your drug dealers. You did not care if she lived or died."
Kane said: "She was the most important thing in my life."
The trial continues.