Captain Brionès was a real brute, he enjoyed mistreating people very much. Proud of his brutality, he encouraged his men to do the same thing and rewarded the best among them with armbands.
Sergeant Garcia did not want an armband. Besides, he was totally opposed to his Comandante's actions. But he was only a soldier forced to obey orders. Fortunately, he knew he was a temporary member of the cuartel of Monterey and that soon he would go back to Los Angeles. That's why, he would not hesitate to put a spoke in his Comandante's wheel.
Moreover, Garcia really enjoyed participating in the arrest of Captain Briones who was publically humiliated when thrown down a well.