What does Proverbs 21:7 mean?

In a broad sense, the "violence" mentioned here implies all forms of sin and evil (Proverbs 6:16–19). Other Scriptures note that those who persist in sin are likely to be destroyed by their own crimes (Psalm 7:12–16; Proverbs 26:27). There is no way for someone to hide their sin from God (Hebrews 4:13) and rejecting His truth leads to eternal ruin (Proverbs 8:34–36).

Of course, because the world is corrupt (2 Peter 1:4), those who do evil are not always caught and punished immediately. Asaph, one writer of psalms, struggled with seeing wicked people succeed (Psalm 73:2–7). Much like this proverb, he described them as wearing violence, speaking evil, and making threats (Psalm 73:6–8). Eventually, Asaph remembered that earthly life is not the end, nor is it the only time for God's judgement to fall. He writes, "Truly you set them in slippery places; you make them fall to ruin. How they are destroyed in a moment, swept away utterly by terrors! Like a dream when one awakes, O Lord, when you rouse yourself, you despise them as phantoms" (Psalm 73:18–19).

Another example of God bringing judgment to wicked people who thought they would escape is found in the end times. The book of Revelation indicates that the Lord will destroy a wicked city or civilization labeled as Babylon. When God does this, a throng of voices praise Him. The voices cry out: "Hallelujah! Salvation and glory and power belong to our God, for his judgments are true and just; for he has judged the great prostitute who corrupted the earth with her immorality, and has avenged on her the blood of his servants" (Revelation 19:1–2).