He couldn't give up.
The battle had exploded from the tiny cracks of rebellion that formed at Hogwarts. Throughout the year, the Slytherins had tried their best to overpower the other houses. In the end, their accomplisments only created more mutiny.
He hadn't spent a torturous 8 months with Snape as the "headmaster" (as if he could live up to the title after Dumbledore) and the Carrows as their teachers to suddenly admit defeat. He had worked too hard with the DA, trying to contiue Harry's teachings in secret to let it all go to waste.
Neville knew that it could have only been a matter of time before the trio returned to Hogwarts. Harry always felt that he needed to rescue others, going as far as putting his own life at risk in the process. The moment Harry, Hermione, and Ron stepped through the tunnel from Hog's Head, the battle was imminent.
He sounded like a military commander when he gave out orders to the DA in preparation for the fight. All the while, he felt guilt commanding them. The students were at the peak of their youth and should've been worried about relationships and grades, not their daily survival. The war had turned them all into soldiers. And like soldiers are wont to do, they all had to keep marching.
That's why when Voldemort announced Harry's death, he knew he couldn't back down. No matter what, he needed to see this through. While insulting Voldemort may not have been the brightest idea, it helped to buy the Light some time.
Seeing Harry's body made Neville even more determined to win the battle. Nagini was slain without a second thought, Voldemort's screams ringing in his ears.
Adrenaline coursed through his body as he fought Death Eaters in the Great Hall. Harry turning out alive brought relief - he knew the end was near.
Neville was a Gryffindor through and through. Despite the bleak circumstances and dark times, he stayed strong.
He endured.