These lines are from D. H. Lawrence's poem "Snake." They represent the internal conflict the speaker experiences as he watches a snake drinking from his water-trough.
The "voices" mentioned are the voices of his "accursed human education." This refers to the societal conditioning and prejudice that have taught him to view snakes as dangerous, venomous creatures that must be killed on sight. These voices are rational, logical, but also fearful and destructive.
The line, "If you were not afraid, you would kill him!" is a taunt from this internal voice. It questions his masculinity and courage, suggesting that his hesitation to act is a sign of cowardice. This creates the poem's central tension between the speaker's instinctual, almost spiritual admiration for the snake as a majestic being and the learned, rational fear that demands its destruction.