What does Proverbs 21:15 mean?

True "justice" is that which corresponds to God's will and His standards (Proverbs 1:1–7; 21:3). When truth and goodness are enforced, through punishment for those who do evil, wicked people are rightly afraid (Proverbs 3:25–26; 10:24).

Not all punishment is just (Proverbs 17:26). Many Christians are persecuted or even martyred around the world. But someday the Lord will execute justice upon the persecutors, and believers will rejoice that the Lord has vindicated them. Jesus said, "Blessed are you when others revile you and persecute you and utter all kinds of evil against you falsely on my account. Rejoice and be glad, for your reward is great in heaven, for so they persecuted the prophets who were before you" (Matthew 5:11–12).

First Peter 4:12 refers to intense challenges to be faced by first-century believers. Scholars suggest he may have been referring to Emperor Nero's practice of burning Christians alive at the stake. Peter tells his readers to "rejoice insofar as you share Christ's sufferings, that you may also rejoice and be glad when his glory is revealed" (1 Peter 4:13). When Christ's glory is revealed, evildoers will be terrified (2 Thessalonians 1:5–10; Revelation 19:11–21).