Hershel's death in the comics occurs during a chaotic outbreak. A group of zombies overrun their location and he gets caught in the fray, ultimately succumbing to the undead.
Another theme could be sacrifice. For example, Hershel might be written as being willing to sacrifice a great deal for Beth's happiness or safety, and this sacrifice is a testament to his love for her. Also, the theme of forgiveness might come up, if there are any misunderstandings or past issues between them in the fanfiction.