@KafukaPalazzo i think he probably did that to pose as he was sorry for losing her thingy, just so he could see her expression of despair while he cut off his horn in supposed atonement
Another example : lets say u have a friend and u keep telling her how beautiful her hair is, then that friend loses something precious to u, and tries to make up by cutting up her hair and giving it to u, how fucked up would that be, in this case this dude did it on purpose, knowing that would fuck her mind up even more.
At least thats what i think, i would be grateful if anyone else can elaborate on this