Mahou Shoujo wo Tasuketai

The protagonist is attacked by a monster and saved by a magical girl. After learning about the existence of magical girls, he begin to take small steps to help protect the city. However, one night, by unfortunate luck, they encounter the mastermind behind it all and learn his plans. The mastermind reveals a plan to intentionally let the monsters be defeated, then hunt down the magical girl after she levels up. The experience points can be obtained simply by stopping the heart. As a result, the magical girl is killed over and over again. The protagonist is horrified when the mastermind speaks about the magical girl's fate, likening it to the taste of the liver of a fat goose. Why does the protagonist try to save the magical girl?