Taken directly from the Zelda page of the "Complete Monster" trope on TV Tropes: "Ganondorf was once a brutal Gerudo warlord who sought to conquer Hyrule during the kingdom's founding days. Desiring the power of the Zonai Secret Stones, Ganondorf murders Queen Sonia after swearing false fealty to King Rauru, and steals her Secret Stone to transform himself into the Demon King. With his newfound powers, Ganondorf creates an army of demons and monsters to sweep over Hyrule and kill all who would oppose him, including his own people in the cataclysmic Imprisoning War until he is sealed away for eons by Rauru. In that timespan, Ganondorf's malice manages to leak through the seal, creating Calamity Ganon, a primal force of evil that brought destruction upon Hyrule for countless generations, with the last one bringing the kingdom's collapse. After freeing himself in the present day, Ganondorf resumes his destructive campaign by unleashing monsters, saboteurs, and natural disasters upon the races of Hyrule, promising to finish what he had started. Faced with defeat by Link and the Sages, Ganondorf swallows his Secret Stone to become the Demon Dragon, sacrificing his mind and body for the power to destroy Link and plunge Hyrule into the "eternal night" of destruction and despair. A narcissistic, sociopathic monster of a man, lacking the redeeming or tragic qualities his incarnations often have, Ganondorf believes himself to be superior to everyone else and thus deserves to rule Hyrule as he sees fit." -Taken Directly from TV Tropes.
(If I tried to explain it, I'd screw it up horribly)