Javier Acosta was the leader of the 'Ghosts', a team who broke into safes and left behind IEDs.
During a job, Agent Simone Taylor and her partner were tailing him, and he and his crew sprayed her partner with bullets and paralyzed him.
Javier, along with his masked group, breaks into a safe factory, takes pictures of a safe, then shoots two works and blows up the safe. His team later robs an armored car, and after a long chase, while attempting to climb down a ladder to escape Walter, the ladder broke and Javier grabbed the ledge with one hand. Walter hesitated in grabbing and pulling him to safety, and had just grasped his hand when Javier fell to his death.
Javier's death shook Walter greatly, and naturally he blamed himself. Even after Sylvester confirmed that he would have pulled Walter over if Walter had grabbed him, Walter still felt guilty.