The word "co-acted" is a verb that refers to the act of working together with someone or something to achieve a shared goal. When looking for antonyms for the word "co-acted," one can consider words that imply working alone or against someone or something else. Some possible antonyms might include words like "opposed," "resisted," "disputed," "defied," or "contradicted." These words all suggest an element of conflict or opposition, indicating that the parties involved are not working together in a collaborative manner. On the other hand, words like "collided," "clashed," or "disagreed" suggest both parties are trying to achieve a similar goal but have differing opinions or methods.