An NYPD sergeant on trial for fatally hurling a drink cooler at a scooter-riding drug suspect took the stand to defend himself Monday — testifying he was trying to save other officers’ lives at the time. The officer had been supervising an undercover buy-and-bust operation on Aqueduct Avenue near West 190th Street in Kingsbridge Heights when prosecutors charge he recklessly grabbed the full Igloo cooler and chucked it at Duprey, 30, as he fled the scene on on his motorized scooter. After being struck, Duprey crashed into a tree and was flung from the bike, cracking his head on the pavement and dying almost immediately on impact.