The second part of this proverb is variously translated,

Wait for the LORD, and he will avenge you (NIV),
Wait for the LORD to handle the matter (NLT),
wait for the LORD, and he will deliver you (ESV),
Wait for the LORD, so that he may vindicate you (NET),
put yi trust in the Lorde, & he shal defende ye (Coverdale).

Save, avenge, deliver, vindicate, defend, handle the matter... such varied interpretations, but the basic idea remains the same: it should not be you who seeks revenge. This matter is for God to settle, or if you prefer, this wrong will be righted by the divine order of the universe, in its own time.

When we have been hurt, or otherwise wronged, there is certainly a tendency in all of us to get our own back, to repay like with like. We hear this expressed in cynical statements such as,

"Revenge is a dish best served cold"¹
"A woman's desire for revenge outlasts all her other emotions"².

Proverb 20:22, in the language of its place and time, is akin to the somewhat calmer and wiser words we have inherited from eastern philosophy,

"Before you embark on a journey of revenge, dig two graves"³

and in more recent parlance,

"If we do an eye for an eye and a tooth for a tooth, we will be a blind and toothless nation"⁴.

The idea that vengeance is the Lord's is a simple, but effective one. It lifts a great burden from our shoulders, and allows us to put our energies into acts of creation rather than acts of destruction.

¹ credited to Star Trek II: The Wrath of Khan
² Cyril Connolly
³ credited to Confucius, circa 500 BCE
⁴ Martin Luther King Jr.