"The black snake" the speaker feels sympathy for the snake once it is killed.
" A Narrow Fellow in the Grass" the speaker feels anxious about the snake.
Symbols can confirm or clarify themes by using something but it meaning something else that reveal the theme, for example, the theme is kindness is often overshined by greed, the symbol is a caged bird without food in a magic castle trapping a selfish, greedy prince. It is only when the prince gets board of the castle but can't go outside because of the magic spell, and realizes the bird also wants freedom, that the theme shows, or is revealed
It means that one should never lose hope because nothing is ever completely dark, there is always something good to which we can cling.