Ram faced the injured Baali. Baali told Ram if he was searching his wife he should have come to him for help and friendship.
Whoever took Sita even if it is Ravan I would have defeated them and would have taken them at your feet and at your mercy.
Baali argued with Ram, why he had to kill him in a cowardly way. Ram explained to him about the various purusharthas and granted him moksha.
Baali was then convinced and also asked his son Angada to stand by Sugriv and assist Ram in the search of Sita.That's how the Vanara Sena came into the picture.