What inspired Valmiki to write Ramyana ?

Today is Valmiki Jayanti, its a celebration in the remembrance of great Saint and Poet Valmiki. He is a great example of evil turning into saint, he was a highway robber called Ratnakara , One day, the great sage Narada, while passing through the jungle, was attacked by Ratnakara. As Narada played his Veena and sang praises of the Lord, he saw a transformation coming over Ratnakara. Then, he asked Ratnakara whether the family, for whom he was robbing others, will partake in his sins also. Ratnakara went to ask the same question to his family and on being refused by all his family members, he went back to sage Narada. Narada taught him the sacred name of 'Rama' and asked him to sit in meditation, chanting the name of Rama, However he only know "mara" which means kill in Hindi. Narada the wise saint then asked Rantakara to recite 'mara' , 'mara' . Ratnakara followed the instructions and kept sitting in a meditative posture for years. 

When Narada came back he found that the transformation happened in the words and he started reciting 'Rama' , 'Rama' in deep meditation. Ratnakara was also covered by an ant hill. 

Narada seeing his penance bestowed him the honor of a Brahmarshi and given the name of Valmiki, since he was reborn from the Valmika (the ant-hill). Sage Valmiki founded his ashram at the banks of River Ganga. 

The death of a bird led to the birth of Valmiki’s Ramayana. Sage Valmiki headed out to take a dip in the river Tamasa. He decided to take a stroll before his bath, soaking in nature’s beauty. A little distance away were two birds, a male and female, completely engrossed in one other. Even as the sage watched, a hunter’s arrow killed the male. Overcome with grief (shoka), Valmiki uttered these words, which, even to his surprise, came out as a shloka – a verse perfect in rhyme and rhythm – and was the first ever Sanskrit verse.

The verse roughly translates to, “Oh hunter, may you repent for life and suffer, find no rest or fame, for you have killed one of the unsuspecting, devoted and loving krauñcha couple.” Of the several interpretations, one says the hunter is the demon Ravana, who separated the loving couple, Sita and Rama.