Question

why does the poet feel that the snake must be killed

Answer 1

As his knowledge of education saying him that the snake must be killed because Snake is dangerous one and can bite any which cause death of some.
So he thaught that the snake must be killed.

Answer 2

Because the voices of his accursed human education repeatedly tell him that the snake is a poisonous one and must be killed for in Sicily the black snakes are poisonous.