The only thing more shocking than the crime itself was the identity of the victim.

At 4.45pm last Monday, on London's famous Oxford St shopping strip, a young man was stabbed to death in a trivial argument over a thrown drink.

Steven Bigby, 22, suffered a single knife wound to the heart and died on the pavement.

There would be no public outpouring of grief.

Bigby, it turned out, was one of 10 men who allegedly raped a 16-year-old girl and poured caustic soda over her.

The known gang member was on bail for the January rape and a brawl involving a knife, but bizarrely police say the crimes appear unrelated to his murder.

Metropolitan police, who arrested two teenagers over Bigby's murder, say he was just the victim of a mindless act of youth violence.

Only hours before Bigby's death, mourners had gathered at a London school to mark the loss of Jimmy Mizen, 16.

Mizen's throat was slashed in a bakery in broad daylight earlier this month, by a youth he had refused to fight. He was the 13th teenager murdered in London this year. The contrast between the two victims couldn't be greater. Mizen was the ultimate innocent; a former altar boy who led an entirely peaceful life.

The two cases highlighted the random nature of London's youth violence crisis. Many families have long feared that no one is safe, and here was more proof.

London's 13 teen murders tell only part of the story. Last year 27 teenagers were killed.

When people under the age of 25 are included, the number of dead rises to 70 since the start of last year.

The proliferation of weapons is stunning.

Earlier this week two officers in a patrol car saw a gang of 24 youths board the top deck of a London bus. A search uncovered six knives, two screwdrivers, a metal baseball bat, claw hammer, corkscrew and a mallet on the group, who were all aged between 14 and 18.

Some of the attacks could be termed "postcode violence". They involve groups brutally punishing outsiders who cross into enter their turf. Often the most violent attacks involve youths trying to prove they are tougher than the rest.

Other attacks, such as the Oxford St and bakery murders, are sparked by minor indiscretions or are unprovoked.

As one youth, 12, put it, when you pass strangers in the street in London you don't look long and you don't look twice.

"I get angry when I have to think about if I'm going to get home safely," he told The Times.

The usual suspects have been blamed: poor parenting, broken homes, violent video games and television shows, alcohol, and poor schooling.

But another theory stands out - disenfranchised youth feel they have nothing to lose. With no prospect of establishing a career or buying a home, these youths are disconnected from society.