Everyone is aware of Killing of Vali.
Sugriva challenged Vali to a fight. And when Vali & Sugreeva are fighting Rama shot Vali from behind.
Now Rama faces the injured Vali. A dying Vali told Rama if you are searching your wife you should have come to me for help and friendship. Whoever took Sita even if it is Ravana I have defeated them and would have taken them at you feet and at your mercy.
These questions arises
What was Vali's crime to Rama?
Even if Vali committed a crime (with his brother), what is Rama's right to kill Vali?
These questions are answered by following ::
The younger brother should be treated like a son. Even if he made a mistake you should forgive him, specially when he promised to respect you for your whole life.
Some people says Vali was not a human so Rama can hunt him like people use to hunt deers etc at that time.
But if we keep Vali in category of animals then what Vali did wasn't a crime. And also at that time monkeys are not hunted by people. And another thing is Vali was a very well known king of his time.
Vali was wrong but same thing is done by Sugreeva after death of Vali. He must have treated Vali's wife as his mother.
So the reasons of Vali killing are not justifiable.
Only good reason is to give respect to boon which Vali have.