Nice idea, simple and effective.

Please find here some suggestions:

1) It would be nice to have a "go to main menu" function.
2) The controls during the "close combat" state are a bit difficult. It might help let the player use the mouse button to fire
3) I think that pressing aim while running should behave the same when you press run while aim, that is walk while aiming. 
 the player press aim while running it keeps running.
4) Consider to implement more GUIs in game to let the player go to the main menu, continue last saved game or start a new one.