Why was Rosa so furious with Matt? Why did she threaten to kill him?

Rosa wasn't so much furious with Matt, as she was with Willum. She wanted a different job... to work in El Patron's home as a housekeep. She didn't want her sole job to be caring for Matthew, something that afforded her nothing more than the laughter of the other servants when they made fun of her. This is what her anger toward Matt stems from... her circumstances. She threatens to kill him because without him her life would be different. Rosa was in love with Willum, but her life belonged to El Patron and the task he'd given her.