Complex sentence: Due to his opinion, he was seen by some as a "traitor" to the political cause, and he felt he could not freely express his art, so in 1926, he decided to leave Mexico and move to New York.
Split into simpler sentences: Due to his opinion, he was characterized by some as a "traitor" to the political cause. Tamayo came to feel that he could not freely express his art; he therefore decided in 1926 to leave Mexico and move to New York.