Lee McCabe, 21, a factory worker, who was to become a father in two months time, was stabbed in the stomach. He staggered about 200 yards before collapsing in front of neighbours and died in hospital soon afterwards.
Police said that Mr McCabe stepped in to stop an argument between his killer and the girl near his home in Camberwell. His brother Pierce said: "It was typical of him to try and help this girl who was a complete stranger to him.
"He saw the girl being slapped by the youth and decided he had to help. When he tried to intervene the man pulled out a knife and shoved it deep into his stomach.
"No one can believe this has happened. It hasn't sunk in. My mother is absolutely devastated by it.
"He spent a lot of time with his girlfriend Emma who is seven months pregnant. He was so looking forward to being a dad.
"He was working weekends to save extra money for when the baby was born and apart from working in his meat packing factory, he also did odd jobs like painting to save for the baby."
Detectives are looking for a man described as aged about 18, 5 ft tall, and slim with blue eyes and black hair. He was wearing a red waist-length jacket, beige jeans and white trainers. He fled from the scene with a friend.Reuse content