In an interview with the Sunday Mail, Athar Shah said that Ahmed laughed in his face during his brother’s dying moments.
He also said that one of his brothers colleagues told him Asad had been attacked.
After hitting the killer with an ice-cream stand, Athar carried his stabbed brother down Minard Road in Shawlands, but could not outrun Ahmed.
He said: “I got as far as a few hundred yards but the monster must have caught up because he pushed me or kicked me or did something while I was trying to contact the ambulance on my mobile.
“I had my phone in one hand and I was trying to carry him.
“I lost my brother there. That monster came and jumped on my brother’s face, right there in the street.
Religious zealot Tanveer Ahmed admitted he murdering the Glasgow shopkeeper because he had insulted Mohammad by declaring himself a prophet online.
The 32-year-old will be sentenced next month but in a defiant call from the city’s Barlinnie jail - later posted on Youtube - he insists his killing was just and that he does not fear his punishment.
Referring to Mr Shah’s family as Qadiani - a derogatory term for their minority Ahmadi branch of Islam, in an Urdu-language statemennt Ahmed says that he is untouched by their pain.
He said: “I stood guard on the honour of Prophet Muhammad and I shall do it again if I shall get a chance.
“My lawyer says that there is a lot of pressure from media, local lawyers politicians and Qadiani family which is mobilising pressure by meeting prosecutors and judges. But I am unmoved. If my God is happy with me then I don’t care if I get 100 years or even death.”
Mr Shah’s family have said they are so upset by the murder that most of them are fleeing Scotland, a country they chose to be free of oppression in their native Pakistan.