Let us understand the THOUGHT PROCESS AND HELPLESSNESS that was the reason for this Adharm. This entire issue is discussed at length in earlier post: RAM SUGRIVE FRIENDSHIP PACT.. AN ANALYSIS .
“Please remember Ram had a better option of calling Ayodhya forces, yet because he wanted that Vaanars should get equal place in the society, he wanted Vaanar Sena. Ram knew if he won the battle against Ravan with the help of Vaanar Sena, it would be far easier for him to bring Vaanars into the mainstream. Ram was certainly NOT gaining anything personally by killing Bali in such a fashion.
And that is the reason why Ram is called Maryadapurushottam. A person who follows Maryadas of his time is called Maryada Purush. But a Maryada Purush, who goes beyond that, and breaks the existing Maryada SOLELY for the sake of Humanity and nation building, is called MARYADAPURUSHOTTAM.”