Her hostility and the forest's cataclysm are explained in flashbacks: when Ori was lost, the Spirit Tree released a flash of light in order to look for him, which killed all of Kuro's recently hatched offspring.
Her hostility are explained in flashbacks. The forest's cataclysm are explained in flashbacks. The Spirit Tree released a flash of light. This was to look for him. Ori was lost. Him killed all of Kuro's recently hatched offspring.