3 small children, baby knifed, 2 dead

What is the mentality of someone who stabs to death a 5 year old, a 4 year old and a six month old baby who may die of its wounds too? Decades ago these things just did not happen and now they are just another daily paragraph in recording the disintegration of humanity.

http://news.bbc.co.uk/1/hi/england/london/7428946.stm

snip

Two children have died and a baby remains in a critical condition after what police believe was a knife attack.
Officers were called at 2230 BST on Friday to Carshalton, south London.
A five-year-old boy, a girl aged four and a six-month-old girl were rushed to hospital with suspected stab wounds. The older boy and girl later died.
A 39-year-old man and a 35-year-old woman were arrested at the scene and taken to separate south London police stations for questioning.