It's cuz after Osiris revealed that Anubis isn't Seth's son but his, Seth started killing women and children like it was Nephthys and Anubis, because of the feels of betrayal and hatred, it has come to the point he became blinded by those emotions and killed "all" women and children, but idk correct me if I'm wrong, I haven't read this in a long time
Seth killed women and children to harvest their suffering and their souls in order to make himself a cursed weapon, the only kind of weapon that would help him protect himself against an undead like Osiris (since he knew, even dead, Osiris still wouldn't stop trying to get him)
Wow, that's darker than I thought (⊙…⊙ )
(⊙…⊙ )
I still don't know what the reason was for all the killing and sins that Seth committed...