One more thing. I don't contest that Rama was justified in killing Vali, and given that Vali had the power to drain out half the power of his adversary, even killing him while unseen wasn't unjustified. However, one contrast between Rama's policy on Vali and his policy on Ravan does strike me.
In Ravan's case, Rama first sent Hanuman, and later Angad, to Ravan's court to persuade him to honorably return Sita, but both were rebuffed. In other words, Rama did everything possible to avert a war before determining that it had to happen.
But in Vali's case, no such attempt was made by Rama to persuade Vali to honorably restore Roma to Sugriv or accept him back into the kingdom, or anything like that: Vali didn't even have a clue that Rama was his enemy. Does one think that Vali deserved even less opportunities than Ravan, and didn't deserve to even know that Rama was his enemy? Being killed unfairly is one thing, but what would seem to aggravate it is being killed by someone who you don't even know has an enmity with you.
Also given that unlike Ravan who tormented the 3 worlds, Vali was someone who kept the vanara race secure when there was no Rama to protect them, and who saw to it that would-be Rakshasha bullies, be it Ravan, Dundubhi, Mayavi, were all shown their place so that they were no threat to the vanaras. In other words, Vali was not guilty of tormenting the earth the way Ravan was, and his only (albeit egregious) sin was to covet Roma.