wise men at their end know dark is right, (line 4)
This is one of the more interesting and surprising lines in the poem. It all depends on what the speaker means by "right." The rest of the poem claims that dying men should struggle against their fate, but here the speaker seems to admit something slightly different – that, if they're wise, dying men know that death is "right." Maybe this means death is appropriate, or natural; maybe it means that death has the right to take them. Either way, it complicates what first seemed like a pretty straightforward text.
their words had forked no lightning (line 5)
No matter how wise you are, it might still be difficult to make your mark on the world. Even if you've spoken great words, they might not have drawn any attention to you.