In Bengal, Diwali is celebrated as Kali Puja, the day that the goddess Kali is worshipped. Kali is hideously ugly and terrifying to look at - but only because she is so angry at the wickedness in this world. Kali is the destroyer of all evil, and is worshipped as such on Kali puja.
The story of Kali that is told to children in Bengal on the occasion of Kali Puja is as follows:
Long long ago, the world was overrun with evil - men had turned to wicked ways, and demons, rakshasas and ogres thrived and prospered. The gods were helpless. They could do nothing to control or contain the evil in the world.
In desperation they turned to the supreme goddess Devi for help. Devi agreed to end the evil, and took on the black and frightening form of the goddess of destruction to do so. This form of Devi is known as Kali, which means 'black'. Devi in the form of Kali then went on a rampage of destruction, killing and destroying all the evil men and demons in the world.
Kali became so angry that she could not stop, even when all the evil had been destroyed. She began destroying the entire world in her fury. The gods asked her to stop, but she didn't hear them. They turned to Shiv her husband for help as the only one who could stop her. But Kali didn't hear him either. So Shiv lay down in her path - and only when she put her foot on him did she come to her senses, and stop her madness of destruction.
Kali is worshipped in her destructive mode. She is terrifying to look at, black and furious, with four hands, dripping blood and dressed in skulls. She is shown with one foot on Shiv who lies prone in her path, and with her tongue sticking out in shock and horror as she realises the destruction she is causing.
Ram, Prince of Kosala
Kali Destroys Evil
See also: Durga Puja