it was shown that vali posess much vighour and capacity than RAVANA. Rama as avatar purusha, knowing every thing in world, could have used VALI too, instead killing him.
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I can't seem to find this information. Vali was a child of the King of Kishkindha; I see no mention that he took birth on earth for the sole purpose of helping Rama.
reply to jill d #170087 querry. Lord Brahma directed "SURAS" (DEAVATALU) and they began to parent sons in the semblance of monkeys (Ramayana 1.17.8). VALI thus as per that directive born to Lord Indra, and as per Ramayana vali is the son of INDRA.
Lord Sree Hari took SreeRaama Avatar to protect DHARMA and to curb ADHRMKA (injustice). Although VAALI is very useful and possess mighty vigor HE had to kill him to uplift the Justice and to show human of era, a true example of what a KSHTRIYA had to do in such circumstances. In Ramacharita, the discussion on DHARMA between Vaali and Sree Raama gained much importance.