Psalms 64:8

So they shall make their own tongue to fall upon themselves - In Psalms 64:3, their tongue is represented as a sword; and here, keeping up the figure, the tongue, as a sword, is represented as falling on them, or as inflicting the wound on themselves which they had intended to inflict on others. This might be rendered, “And they have cast him down; upon them is their own tongue;” or, “Upon them their own tongue has come.” That is, someone would cast them down, and they would fall as if smitten by their own tongue like a sword. It is not said who would do this, but the most natural interpretation is that it would be done by God. The idea is, that the instrument which they had employed to injure others would be the means of their own ruin.

All that see them shall flee away - Compare Psalms 31:11. That is, they shall flee in consternation from those who are so fearfully overthrown. They shall see that God is just, and that He will punish the wicked; and they will desire to escape from a ruin so dreadful as that which comes upon the ungodly. The idea is, that when God punishes sinners, the effect on others is, and should be, to lead them to wish not to be associated with such people, but to escape from a doom so fearful.