The "usual suspects" for a 20YO dying would be violence, health problem, accident, suicide, and drugs. Apparently "foul play" has been ruled out, and his having served (or serving) in the Navy suggests his health was OK. So accident, suicide, or drugs seem to be the most likely possible causes. Other than he and Hill possibly having had a messed up upbringing, I wouldn't be inclined to his death to her antics and downfall.