Why does God let the righteous suffer while sinners live freely?

If you're talking about those who are walking righteously in the Lord, this is also David's dilemma when he sees the evil people living as if they're not sick and even flourishing. He gradually realized, however, that the fate of those who rebelled against the Lord would be horrible someday. As the great Psalmist writes, “Fret not yourself because of evildoers; be not envious of wrongdoers! For they will soon fade like the grass and wither like the green herb” (Psa. 37:1-2). Asaph also writes in the same way, “But as for me, my feet had almost stumbled, my steps had nearly slipped. For I was envious of the arrogant when I saw the prosperity of the wicked… But when I thought how to understand this, it seemed to me a wearisome task, until I went into the sanctuary of God; then I discerned their end” (Psa. 73:2-3, 16-17). Suffering is occasionally experienced by the righteous in order to increase their faith and relationship with Christ. After all, knowing Christ in our lives is more important than anything else in this world (Phil. 3:8).