Heat of Passion Murder

William is a single parent of a seventeen-year-old son. William went out to dinner one night , leaving his son at home with instructions that he was to have no visitors. When William returned home, his son was hosting a party for several teenagers. William angrily told the guests to leave, getting into an argument with one by the name of Bobby. Bobby left but returned a half hour later and threw a beer bottle through William's living room window. William didn't know what had broken the window until much later, but he heard the window break and saw the glass shatter. He ran outside with the handgun that he kept in his house for protection. Bobby had parked his car on the street in front of the house. As Bobby was starting to drive away, William shot once from the front porch, hitting Bobby in the head and killing him.

Has William committed homicide? If so, what degree of homicide?