Why this knife-wielding man named Qiang, killed one person, injures 6 others in China?

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Here's the news,
Knife-wielding man kills one,
injures 6 in China
Beijing, Jan 3, 2013, (PTI) :
One person was killed and six others were injured when a 21- year-old knife wielding man went berserk in Chinese city of Shenzhen, randomly attacking
people by slashing at their faces.
Video footage posted on social website showed the man armed with fairly long knife first attacked a woman walking along with her friend on the footpath yesterday in Baoan District of central town and cut her nose off.
As people began fleeing in panic he
targeted another woman slashing at
her face and targeted people who
came his way, specially their faces and
collars.A middle aged man died of wounds and six others injured, a media report said today.
The young woman whose nose was
cut off was being treated in hospital.
The attacker, who was identified as
Qiang, was later arrested by police. He
was stated to be angry over not finding a job.
Such random attacks, stated to be
social revenge assaults were reported
from all over China in the recent past.
The attackers, often described by official media as people with mental
health problems targeted children in
the schools On December 25, a man drove his car right into a group of 23 students at middle school at Manchu
Autonomous County of Fengning,
Hebei province in a bid to mow them
down. Thirteen students were injured in the attack. He was stated to be angry over the murder of his daughter.
On December 14, a man armed with
cleaver attacked children in a village in
primary school in China's Henan Province, injuring 23 of them.
He was stated to be suffering from
epilepsy and disturbed by doomsday
rumours that the world would come
to an end by Dec 21.

what are the reasons behind these type of cruel behaviors?Are they fed up with their life and country?