Njáls saga explores the consequences of vengeance as a defence of family honor by dealing with a blood feud spanning some 50 years. The saga shows how even worthy people can destroy themselves by disputes and demonstrates the tensions in the Icelandic Commonwealth which eventually led to its destruction. Any insult to one's honor had to be revenged: sometimes this includes slights which seem trivial to modern readers. Magnus Magnusson finds it "a little pathetic, now, to read how vulnerable these men were to calls on their honour; it was fatally easy to goad them into action to avenge some suspicion of an insult".
Insults involving a character's manliness are especially prominent in the saga. Thus, Njáll's lack of a beard is repeatedly referred to and used by his opponents to call his manhood into question. Another example, among many, is when the gift of a silk garment is considered an insult by Flosi and a hard-won settlement breaks down as a consequence. Ármann Jakobsson has argued that it is "difficult to find a man whose manhood is not vulnerable" and that Njáls saga criticizes the idea of a misogynistic society by showing that the ideal of masculinity can be so restrictive that it becomes oppressive to men and destructive to society.
Omens, prophetic dreams and supernatural foresight figure prominently in Njáls saga. The role of fate and, especially, of fatalism is, however, a matter of scholarly contention. Halldór Laxness argued that the saga is primarily a book about the fatalism inherent in Norse paganism. In his view, the course of events is foreordained from the moment Hrútr sees the thieves' eyes in his niece and until the vengeance for Njáll's burning is completed to the southeast in Wales. In this way, Laxness believed that Njáls saga attested to the presence of a "very strong heathen spirit", antithetical to Christianity, in 13th century Iceland. Magnus Magnusson wrote that "[t]he action is swept along by a powerful under-current of fate" and that Njáll wages a "fierce struggle to alter its course" but that he is nevertheless "not a fatalist in the heathen sense". Thorsteinn Gylfason rejects the idea that there is any fatalism in Njáls saga, arguing that there is no hostile supernatural plan which its characters are subject to.