The characters of Sugriva and Vali needs to be studied in detail to understand Rama’s behavior towards Vali. When Kabandha describes Sugriva it is full of praise.
The words he uses are Your mission to find Seetha is impossible if you do not befriend Sugriva [3-72-10]; (3-72-12 to14)Self-respectful (आत्मवान्) ; Valiant (वीरः) ; Masterful amongst Vanaras (Monkeys) (वानरेन्द्रो) ; Mettlesome (महावीर्यः) ;Self-resplendent (तेजोवान्) ; ill, limitable, in self-irradiance (अमित प्रभः) ; Honest (सत्य संधो) ; Culture bound (विनीतः) ; Task master (धृतिमान्) ; Mastermind (मतिमान्) ; Masterhand (महान्) ; Capable [adventurer](दक्षः) ; courageous [exploiter] (प्रगल्भः) ; coruscating [in personality] (द्युतिमान्) ; Brave(वीर) ; incomparably intrepid, incursive (महा बल परा) ; Great souled (महात्मना) ; Your (ram’s) helpful friend (सहायो मित्रम्) ; elephantine – straightforward (कुंजरः) ; conversant with all of the strongholds of anthropophagite demons (अशिनाम् लोके नैपुण्यात् अधिगच्छति)
With such excellent references no wonder Rama opted to go to Sugriva instead of Vali. Kabhanda says only one word about Vali i.e. क्रुद्धेन angrily. So nothing bad is said about Vali. Here Valmiki makes a devious course of showing us the character of Vali ( All in the words of sugriva So Biased?). Vali is shown as father’s favorite (पितुः बहुमतः) . Then it is incidences from which we are supposed to draw on Vali’s character. When Maayaavi challenges Vali he immediately rushes in spite of everyone including Sugriva dissuading him. When Maayaavi runs away he follows him chases him. When Maayaavi goes into crater covered with grass he orders Sugriva to wait there, and rushes into the hole chasing Maayaavi. For one year he chases Maayaavi and finally kills him. And during all this time he expects Sugriva to stand guard. When he finds Sugriva has assumed he is dead and is crowned as king gets angry and outcasts him from the kingdom. While chasing Maayaavi he not only kills him but proudly announces he killed his relatives also [4-10-20]. He expected Sugriva to stay there as without his help he will not be able to find the way out. And for not standing guard he attacks his brother. Similarly when Dundubhi challenges him he is given time to recover from drinking, but sets into fight immediately. When Sugriva challenges him for the second time Tara his wife pleads and warns him, even cajoles him to settle the matter with Sugriva amicably. All in vain. Thus Vali is projected as a character with immense strength in the hands of an evil person who causes misery not only to his enemies but also to his near and dear ones. The Rama’s behavior towards the brothers is mainly based on these aspects. When in need, choose a reliably and honorable friend not a strong one.
All Sloka Translations are from the site Valmiki Ramayana