So group B don't live on the island and are invading it to steal group A's food (and kill them if they get in their way)?

Then group B are invaders, aggressors. Group A need the bananas to survive. Defending their home and their survival is self defense.

Group B are trying to implement a hierarchy where they can invade other people's home and take their food by force. They will colonize and kill and probably enslave the islanders. B are pro-hierarchy for sure. A are just defending themselves from invasion.