enemy, revenge
讎 centers on a hostile relationship: an enemy or the act of seeking revenge against one.
讎 combines 雔 (a pair of birds, suggesting a matched pair or opposition) with 言 (words), likely expressing the idea of answering back or matching an opponent in words, which extended to enemy and revenge.
Two birds (雔) facing off with words (言) between them: imagine a pair of angry birds squawking at each other, representing an enemy or a grudge that demands revenge.
For シュウ, picture an enemy shouting 'shoo!' at the pair of birds: shoo -> シュウ, and the birds become your rivals.
revenge; bitter enemy