"And yes, Carter, Horus, whatever you call yourself, you were so mad, you cut off her head. You were reckless, you see—about to charge Set while you were still weak, and Isis tried to stop you. That made you so angry you took your sword—Well, the point is, you almost destroyed each other before you could defeat Set." (23.135)
If Horus gets so angry that he cuts off Isis's head as revenge for delaying his fight against Set, well, that's bad news. Especially considering that Horus and Isis are supposed to work together to defeat Set in this newest incarnation, too.