This is a passage from Ｒｅ：ゼロから始める異世界生活 2 light novel. Subaru has finished telling Ram the Japanese story The Ogre who cried. story summary by Crunchyroll (link contains spoilers for the anime series)
Put simply, the story is of two friends who just happen to be ogres, one red and one blue. The red wishes to befriend the people of the nearby village, but they fear him and see his attempts to befriend them as tricks. The blue ogre comes up with a plan in which he pretends to attack the village, telling the red ogre to rush in and save the day by chasing him off. It works perfectly and the village welcomes the red ogre, who becomes very happy. After some time, the red ogre begins to wonder what became of the blue ogre, so he travels to his house and finds a letter left by the blue ogre, who has gone away so the village won't discover the two ogres friendship. Realizing his friend has left forever, the red ogre weeps.
Ram is not happy with the behavior of Red and states her opinion plainly:
I can’t quite figure out the exact meaning of
身を切る here. It seems to be used mainly in the figurative meaning of “cut (the body/flesh) like a knife”, e.g. (from EDICT)
身を切るよう - piercing (cold), biting (wind)
But here it looks to be used in the literal meaning? Is she saying “he should have cut his body (to cut off horns) before Blue had to leave”? Or is there some other interpretation I’m missing?