Psalm 129

 129 (Biblical text in italics)

1“Many times they have assailed me from my youth up,”
Let Israel now say, Perhaps a responsive reading?

2 “Many times they have assailed me from my youth up;
Yet they have not prevailed against me. They remember how often Israel has been in trouble and that God has preserved them anyway.

3 The plowers plowed upon my back;
They lengthened their furrows.” There has been definite suffering, slavery even. 

4 Yahweh is righteous; He keeps His promises because God is good, no matter how we suffer.  By remembering past trials, we can trust God because we know how He has delivered us from them and stood by us while we dealt with them.  (See the book of Job, especially Job 13:15a.)
He has cut up the cords of the wicked. He is just and saves them from slavery.  Ultimately, He saves us from the slavery of sin.

5 Let all who hate Zion
Be put to shame and turned backward;

6 Let them be like grass upon the rooftops,
Which dries up before it grows up;

7 With which the reaper does not fill his hand,
Nor the binder of sheaves the fold of his garment;

8 And those who pass by will not say,
“The blessing of Yahweh be upon you;
We bless you in the name of Yahweh.” This is a prayer for those who are enemies of Israel to suffer also or at least to fail in their purposes and bear no fruit.  (The following are notes from the ESV Study Bible paraphrased.)  This is not vindictive because 1. It is realistic - to protect us, God thwarts our enemies; 2. It is just; 3. It takes God at His Word; and 4. It is an appeal based on faith, not a need for vengeance.