In the transgression of an evil man there is a snare (Proverbs 12:13). The snare is that the sinner is caught and held fast by his sin, and cannot escape, as he knows nothing of repentance, and has no will to cast off evil habits (Proverbs 24:16).
(For "snare," comp. Proverbs 18:7; Proverbs 20:25; Proverbs 22:25.) Septuagint, "For a man sinning there lies a great snare." But the righteous doth sing and rejoice. The antithesis is not very obvious.
It may mean that the good man has a conscience at peace, is free from the snare of sin, and therefore is glad; or that, in spite of a momentary fall, though he has transgressed, he knows that God forgives him on his repentance, and this makes him happy; or, generally that he rejoices in the happy life which his virtue procures for him here and hereafter (Matthew 5:12).
In the original "sing" represents the sudden outburst of joy, "rejoice" the continued state of happiness. "The righteous shall be in joy and gladness ( ἐν χαρᾷ καὶ ἐν εὐφροσύνῃ)," Septuagint.