Imagine post-war Harry using time turner to come back to october 31st 1981 to save Lily.
His live purpose is ended. He killed Voldemort. So what now?
He sees happy couples all around and can't help but think about the possibilities taken away from his parents.
He's 22. He's officially older then his parents ever got to be.
So he embraces his Slytherin side and schemes.
He gets the job in Department of Misteries and by that gets the access to the time turners.
He transports himself to that cursed day, the October 31st of 1981.
He sees that the door is already open.
He runs past James' body.
He's upstairs and sees Voldemort starting to cast Avada.
He doesn't have time to do anything.
So he does the same thing that Lily Evans-Potter, his mother, did for him over twenty years ago.
He jumps in front of her, shelding her from the curse.
The green light hits him but before his death, before his heart stops, he curses back and sees Voldemort dying all over again.
He know that it's not enough, not with horcruxes that he still has.
And he's dying but that's okay.
Because his mom gets to live.