Vali was famous for the boon that he had received, according to which anyone who came before him lost half his strength to Vali, thereby making Vali invulnerable to any enemy. Nobody can face him directly. As a human being Lord Rama is not an excption to general rules. So it is inevtable for Rama to resort to such a step.
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Vali was having a blessing than he will get double energy of opponent engage in combat and since bhagwan ramji wanted to help his friend that is why he adopted this method as he is ready to take any blame to help friends and to kill evils.