"I have something to expiate"....explain.....

URGENT exam tomorrow!!

as i said in your previous question - "to expiate" means to make amends. the poet regrets his action of striking the innocent snake. the snake had been harmless and dignified. trying to kill it was a mean act and the poet wishes to make amends for his vulgur, petty, and mean action. he wishes to give due honour to the snake that it deserves.