A níðingr is a person regarded as dishonorable, shameful, or untrustworthy. The word belongs to the language of reputation: it marks someone whose conduct has made them contemptible in the eyes of the community.
In a campaign, níðingr should not be used for every enemy. It fits betrayal, oath-breaking, cowardice that endangers others, murder under guest-right, abuse of the helpless, or public refusal to accept responsibility after a grave wrong.