...you know, i'm ok with him sparing the mom. Not because she derserves to live, but for why he did,
"...she resembles you. Why didn't I notice..."
Had he said something like "she wouldn't want me to do it" or "you are not worth dirtying my blade", it would have just been his own arrogance at presuming to know what philomel wanted (he was right, but he didn't know that) or just him thinking he's above killing the womam who stole his child. This way, however, shows how much philomel means to him and how much her disappearance has him shook, to the point he can't even kill a woman who looks like her, even if she deserves it