This could be fun if you could adjust shooting directions at least while not having a zombie in range. As soon as you have fast zombies it is awful to have to run towards him.
And why would a character be able to aim behind but not shoot there? Maybe you could have aim directions turn with walking directions.
Also indicate how soon characters can shoot again.
Do not have some strange pop-up hiding a huge part of the screen closable by mouse only and the character reacting to the corresponding closing click.
Make the tutorial screen accessible from the main menu.