(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation. Before accounting for this rendering, I will analyze the plain sense of the אִישׁ terms, by employing a situation-oriented construal as outlined in “Notes on Gender in Translation,” pp. 11–16.)
Given that אִישׁ prototypically relates its referent to the depicted situation, אִישׁ here it readily profiles the referent situationally: as opponents who are construed as a group (“collective”), in a conflictual situation. Their presence is essential because it defines the situation. See further my comment at Joshua 10:24.
The reference is not nonspecific as it is usually understood, but rather contextually unique. The petitioner was saved from actual opponents—not hypothetical ones. In a case of opposition, it can go without saying that the category denoted by אִישׁ has only one member; and in Hebrew, a contextually unique reference needs no determiner. (Other instances of bare אִישׁ with a contextually unique reference include Gen 2:23; 4:1; 38:25; Judg 18:25; 1 Sam 10:22; 2 Sam 3:15; 4:11; 18:20.)
Elsewhere (Ps 140:2, 12; Prov 16:29; 3:31), this noun phrase is associated with slander and false accusation. Alison Ruth Gray’s monograph Psalm 18 in Words and Pictures: A Reading Through Metaphor (Brill, 2014), pp. 183–85, discusses this expression and concludes: “Within the literary context of 2 Samuel, reflecting on David’s life and his various battles, איש חמס could certainly be understood alongside מריבי עם in v. 44 as a reference to troublemakers on the ‘inside’, to ‘calumniators’” (p. 184; citing the rendering by Dahood in his Anchor Bible commentary, 1966:119). I.e., the enemies in view are both foreign and domestic.
Given that the Bible repeatedly goes out of its way to show that David has the strong support of various women, I infer that the political opinions of women do matter and thus would be taken into account if they turn hostile—as the case of Michal demonstrates (2 Sam 6:20–23). Therefore women cannot be excluded from view as part of the opposition.
As for rendering into English, the NJPS ‘lawless men’ nowadays has a masculine cast. Yet there is no warrant for rendering in gendered terms. The most accurate rendering option nowadays is a role term that more explicitly alludes to the situation; see further my comment at Joshua 10:24. For ‘the opposition’ as a situationally accurate rendering of אִישׁ, cf. NJPS ‘opponent’ at 1 Kings 20:20.