Honestly mate, rereading the Forerunner Trilogy is super rewarding. I'd recommend you reread it just cause it makes a little more sense every time. To answer your question, the Precursors BUILT. They were easily capable of destruction and fighting, but theyd been around since.. Ever really, and never had any known reasons for taking up arms. After they created the Forerunners, they decided that their Mantle of Responsibility would have to be held by another, (Humans) but in the meantime, the Forerunners shall uphold the Mantle. Forerunners were arrogant, and much to the surprise of their creators, decided to fight back the Precursors and succeeded to do so, however this changed the entire outlook of a lot of the Precursors (having their own creations turn on them) so they vowed never to build anything again but instead take revenge. So some fled to the outer reaches of the Galactic Rim, and some turned themselves into a powder with corrupted after time and became the Flood. I hope that helps (and I hope my info is correct)