Superman nearly out of gas decided to go all-out, and took DD out with him.
In the story, Superman wasn't "out for the kill" until he realized it was truly, "kill or kill be killed". By the time he realized the threat Doomsday represented Superman was already severely damaged himself. Had he gone for blood from the start like a Black Adam character would have, he would have IMO, finished Doomsday faster and survived the battle.
But the story was "The Death of Superman" and it would have also been extremely out of character for Superman to go for the kill right away.