MAHOU SHOUJO WO TASUKETAI

Description

The protagonist is attacked by a monster and his life is saved by a magical girl.
Learning of the existence of magical girls, he begins to keep a watchful eye on the town, however weak he may be.

But one night.
He unfortunately encounters the mastermind and learns the ending of the story.
He deliberately lets the monster die, and then hunts the magical girl, who has leveled up.
Experience points can be gained simply by having the girl's heart stop. That's why the girl gets killed over and over again.
He is frightened by the mastermind telling him the end of the magical girl's life, as if imagining the taste of fat goose liver.

Why does the protagonist save the magical girl?