Laslow's fate was sealed the day he shot a small bird with his slingshot. He was only 9 so the disguised demon he nearly killed, allowed Laslow to live for his offence. On the condition that on his 27th birthday, Laslow would cease to be human and would merge with the demon for ever. On that day he said farewell to his family and human life and became a bird demon himself.
The demon is not evil, just fair.