Question

Why didn’t lord Hanuman help Sugriva to kill Vali?

Answer

Be Happy is right in answering this.

Shri Hanuman ji was certainly more powerful than Vali as well as Ravana. This fact is supported here, go through 35th and 36th sarga for details in Valmiki Ramayana. enter image description here

The text in image translates as (this is my translation, not the literal one):

Shri Rama: Undoubtedly, there was no match of the strength that Vali and Ravana possessed, but i believe that even the strength of both of these put together cannot be matched with the strength of Hanuman.

enter image description here

The text in above image translates as (again my translation):

Shri Rama: When Vali and Sugriv became enemy of each other; then as forest fire destroys trees, why did Hanuman not destroy Vali in the same way? This is difficult to understand. I believe that Hanuman did not remember his strength at that time. That is why, he was left just to see his beloved Sugriv suffering in such a situation.

enter image description here

The translation is:

Shri Agastya Rishi: O' Shri Rama! Whatever you say about Hanuman is all true. There is no match to Hanuman in strength, intelligence and speed. O' destroyer of enemy, Shri Rama! Sages, whose curses are never vague, had cursed Hanuman that despite having the strength; he would be forgetful of the same.

The link is about conversation between Rishi Agastya ji and Shri Rama in Uttar Kand.

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