In Ramayana, Lord Rama gave a chance to Ravana to surrender and sent Angada as his ambassador for peace conciliation. This was because mercy befits a warrior.
Why didn't Rama, for the same reason, give a chance to Vali to surrender and give Sugriva his rights to the throne of the Vanara kingdom?
Vali was given enough of chances by Sugriva. Sugriva wanted peace, but Vali denied this. Lord Rama knew this, so there is no point in giving Vali another chance.
Further, to follow dharma, Lord Rama also gave Vali a chance to fight with bravery against Lord Rama himself. He had promised Vali that he will be given a chance to avenge his unjust1 murder. See the following quote,
It is also said, Rama promised Vali to give him a chance to avenge his unjust murder. Vali was reincarnated as the hunter and archer Jara in Mahabharata (Dwapara Yuga). Jara was the cause of the death of Shri Krishna (the re-incarnation of Shri Rama) when he struck His feet by an arrow taking them to be a deer.
Source - Do any scriptures say that the hunter Jara was a rebirth of Vali?
Moreover, Ravana deserved a second chance because he was a Brahmin, he had pleased Lord Shiva with uncomparable worship. He was an extensively genius person who had knowledge of all scriptures. It was worthy to consider forgiving him and let him stay on Earth. But anyways, Ravana was destined to be killed by Lord Rama.
1. Unjust according to Vali himself, and not according to the laws of nature.
It was a hostage crisis like Kandahar.
अभार्याः सह भार्याः च सन्ति अत्र वन चारिणः | लुब्धेभ्यो विप्रलब्धेयः तेभ्यो नः सुमहद् भयम् || ४-१९-१६ "There are foresters in Kishkindha, some with wives and some without, some hankering after wives, some whom we have victimized, and from them, we have very great danger." So said monkeys to Tara. [4-19-16]
Even in the capital, many followed the suite of Vali in incarcerating other's wives. Now they all may turn up since Sugreeva won the battle, assaulting such followers of Vali's style of living. This is the fear of individuals reported to Tara.
The ONLY option was a covert operation.Dharmaakuutam, the famous commentator explained to us.
When it is said by Rama that 'today only I will kill Vali...' then it may be countered by saying, 'then why killing him from distance, why not confront him?' If Rama comes to fore, fear may grip Vali, then he may take Ruma, Sugreeva's wife, and to insult, Sugreeva further, Vali may go to a distant place with her. Or, he may seek shelter with Ravana. Or, he may even take refuge in Rama like Sugreeva. Or, on seeing Sugreeva with enough support, he may summon all his Vanara army to fight with Sugreeva and his supporters. Then it will prolong for a time. Then the promise of Rama made to Sugreeva to accord his wife and his kingdom by killing Vali, also prolongs. Justice delayed is denied. Otherwise, if Vali surrenders to Rama, as the killing of a refugee is not a merited act, Rama has to pardon Vali. Whether Vali takes refuse or not, killing Vali on that day itself, and establishing Sugreeva in Kishkindha are the promises made by Rama, at the time of befriending Sugreeva. The word given is to be kept up. So Rama did it and there is no unrighteous deed done by Rama. And Vali also accepts this in the coming stanzas.
It would have prolonged the entire process to restore Sita. It would have jeopardized the safety of other women in Kishkindha.Jeopardizing life of women in Kishkindha and prolonging process to restore his wife to some misplaced sense of self-righteousness to demonstrate valor is illogical.
Therefore it is fair to conclude that Rama picked the most logical alternative.
