Well, a little into Halo lore.
Basically, when the human-forerunner war takes place, it is caused by humans attacking certain cities on Forerunner planets, or some planets entirely, glassing them as the Covenant did with Human planets later. We later learn However, that the humans were eliminating flood in those cities and had to destroy the entire city to prevent contamination. Then, the Forerunners, not knowing that the humans were doing this to stop the flood, made war with the humans, and the war began. The Librarian finds this out later in the war, and then, the Forerunners came into contact with the Flood on a large scale, beats the humans, lose to the Flood. Knowing that the Forerunners will lose and the universe will be contaminated, The Librarian carries out her plan to give the humans a shot at fighting the flood and holding the mantle. She "restarts" everything, and knows that the humans will be better mantle holders than the Forerunners, who has previously prevented any race from developing too far, as they did with the humans.