To testify against him that which is wrong; literally, to testify against him defection, i.e. from the Law of God. The speaker has apparently in view here all such defections from the Law as would entail punishment on the convicted offender.
In Deuteronomy 13:5 (6), indeed, the crime described here as "that which is wrong" (margin, "falling away") is specially the crime of apostasy to idolatry; but the word ( סָרָה), though usually expressing apostasy from Jehovah, has properly the general sense of a deflection from a prescribed course (from סוּר, to go off, to go aside), and so may describe any departure from what is constituted right.