Responses
Ra wanted to overthrow Osiris, so if she did that, it was probably because she knew Osiris was secretly turning Egypt into a desert, and needed to be stopped (pity no one else realized). Osiris was doing that to keep Seth close to himself. Making Seth the god of war and marry Nephthys was all also so that Seth would stay based in the royal palace, under the same roof as Osiris.
Ra...wanted something from Egypt?
And Seth never wanted to be a war god?