Because he had finally realized that the plague was stupid and unnecessary therefore he was saying a plague on both your houses as in I hope something bad happens to you guys so you can regret all this
“It could eat all the streams we know and still be thirsty. Its name is Ou-dis-sun, the Sacred, the Long.”
“Toward the setting of the eighth sun, I came to the banks of the great river.”
Cause they both want to be the leader of the team easy as that