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.
Kishkindha Kanda Sarga 19
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
Kishkindha Kanda Sarga 18
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.