Albert Barnes Commentary Psalms 91:8

"Only with thine eyes shalt thou behold, And see the reward of the wicked." — Psalms 91:8 (ASV)

Only - That is, this is “all” that will occur to you. The only thing which you have to anticipate is that you will see how God punishes sinners.

With thine eyes shalt thou behold and see the reward of the wicked - Your own eyes will see it. (See the notes at Psalm 37:34). You will see the just punishment of the ungodly, the vicious, the profane, and the sensual. You will see what is the proper fruit of their conduct and what is the just expression of the views God takes of their character.

This undoubtedly refers to the general principle that there is a moral government on earth, that vice is often punished as such, and that the general course of the divine dealings shows that God is favorable to virtue and opposed to vice.

The system is not complete here. Indeed, there are many things that could not be reconciled with this if the present world were all and if there were no future state. However, the course of events indicates the general character of the divine administration and the tendency of things.

The completion—the actual and perfect adjustment—is reserved for a future state.

The facts as they occur on earth prove that there is an attribute of justice in God. The fact that His dealings here are not wholly and fully in accordance with what justice demands further proves that there will be a state where full justice will be done and where the whole system will be adjusted.