The speaker of the poem says that reason rarely guides people, or sways them to act in considered ways. Entire nations are destroyed because of the schemes and decisions of fools who are guided by a desire for vengeance, not by reason.
What's more, fate has a way of turning every wish, every gift of nature, and even the gift of art into something bad (this speaker isn't the most optimistic guy around, clearly). Even qualities like courage and elocution (clear speech) are corrupted and used toward bad ends.
In these lines we can see the use of a couple of literary devices. There's our old pal metaphor again, when the speaker refers to the pains of fate as a flying dart ("wing[ing] with ev'ry wish th' afflictive dart").
There's also consonance, particularly in the lines "How nations sink, by darling schemes oppress'd,/ When vengeance listens to the fool's request," which repeat the S sound throughout. Check out "Sound Check" for more on that.