He has saved humanity finally, just dies because he has no further purpose. Now to the real answer: He slowly gets up after falling due to fatigue. He knows this is it, his last chance to save humanity, again. The flood are truly gone, the covenant completely disbanded, all except the tired and injured forerunner and a small handful of knights. Standing there, he pulls out and looks at the AI chip, (from the trailer), and has a Cortana flashback. He focuses again when a knight shifts, and with the almost empty rifle in his hands, the grenade in his one side, half empty pistol on his other, he charges. Between quick flashes of memories of cortana, he dispatches the first few knights. He stabs another knight with the empty gun, abandoning it, while reaching for his pistol in a quick, smooth motion. He takes out 2 more with the pistol, and the last with his knife. With knife in hand, he watches the Didact, wary of his power. Chief charges, the Didact responds with a charge of his own. They're only feet away when the chief reaches for the special grenade on his side, a miniature nuke made for this mission alone. Chief pulls the pin, and for a moment, the Didact, the world, and himself, seem to freeze in time. And as the final memories of Cortana's final moment with cheif fades, the world turns white, and time starts again, only to end a second later. When scans find no remains of either the Cheif or the didact within the crater, the UNSC-Sanghellie alliance agrees to leave the site untouched, a final tribute to the last Spartan-II. Lasky and others go to pay respects, leave symbolic objects behind, and say their last farewell. (Sad music plays, and screen opens to Lasky telling everyone onboard the Infinity over holo screens, humans and Sanghelli alike, the legend and story of the Cheif, and what he gave in order to protect, while it is inscribed in metal at the crater as well. Screen fades, credits roll)