BDAG have very little to say about κατάλαλος:
κατάλαλος, ον (s. two prec. entries) pert. to speaking ill of others, slanderous w. δίψυχοι Hs 8, 7, 2. W. δόλιοι 9, 26, 7. Subst. ὁ κ. (POxy 1828 r, 3) slanderer (in a list of vices) Ro 1:30 (κακολάλους D); Hs 6, 5, 5.—DELG s.v. λαλέω. TW.
Arndt, W., Danker, F. W., Bauer, W., & Gingrich, F. W. (2000). A Greek-English lexicon of the New Testament and other early Christian literature (3rd ed.). Chicago: University of Chicago Press.
Exported from Logos Bible Software, 5:40 AM June 30, 2019.
James forbids κατάλαλος saying that when one κατάλαλος a brother we speak evil of the Torah:
[Jas 4:11-12 NKJV] (11) Do not speak evil of one another, brethren. He who speaks evil of a brother and judges his brother, speaks evil of the law and judges the law. But if you judge the law, you are not a doer of the law but a judge. (12) There is one Lawgiver, who is able to save and to destroy. Who are you to judge another?
[Jas 4:11-12 MGNT] (11) μὴ καταλαλεῖτε ἀλλήλων ἀδελφοί ὁ καταλαλῶν ἀδελφοῦ ἢ κρίνων τὸν ἀδελφὸν αὐτοῦ καταλαλεῖ νόμου καὶ κρίνει νόμον εἰ δὲ νόμον κρίνεις οὐκ εἶ ποιητὴς νόμου ἀλλὰ κριτής (12) εἷς ἐστιν ὁ νομοθέτης καὶ κριτής ὁ δυνάμενος σῶσαι καὶ ἀπολέσαι σὺ δὲ τίς εἶ ὁ κρίνων τὸν πλησίον
I'm wondering if καταλαλεῖτε ἀλλήλων might have a forensic association indicating that this refers to an accusation in a Roman court. By appealing to a Roman court one implies that the Torah is not adequate for resolving issues among the saints, making this largely parallel with Paul:
[1Co 6:1-8 CSB] (1) If any of you has a dispute against another, how dare you take it to court before the unrighteous, and not before the saints? (2) Or don't you know that the saints will judge the world? And if the world is judged by you, are you unworthy to judge the trivial cases? (3) Don't you know that we will judge angels -- how much more matters of this life? (4) So if you have such matters, do you appoint as your judges those who have no standing in the church? (5) I say this to your shame! Can it be that there is not one wise person among you who is able to arbitrate between fellow believers? (6) Instead, brother goes to court against brother, and that before unbelievers! (7) As it is, to have legal disputes against one another is already a defeat for you. Why not rather be wronged? Why not rather be cheated? (8) Instead, you yourselves do wrong and cheat -- and you do this to brothers and sisters!
Does κατάλαλος ever appear with association with a secular forensic setting? Without extant examples of a forensic use of the word is it reckless to posit such a usage in analyzing the text or does it seem unlikely, perhaps because extant examples reliably use a different term?
What I lack in youth I make up for in immaturity.