Complex sentence: He began to focus not only on portraits of the main protagonists (such as Francisco Villa) and general battle scenes, but on executions and the dead; he focused on people whose faces showed such expressions as pain, kindness, and resignation.
Split into simpler sentences: He began to focus not only on portraits of the main protagonists (such as Francisco Villa) and general battle scenes, but on executions and the dead. He focused on people whose faces showed such expressions as pain, kindness, and resignation.