Though they know someone is going to be killed, the people still take part in the lottery enthusiastically , why is it so?

I think conformity is at issue here. Jackson not only demonstrates the power of conformity, given that none of the townspeople protest or question the ritual, but also the human capacity for mindless brutality and evil. "The Lottery" takes the theme of conformity, as found in "Flower Garden," to its violent extreme. No one in the town is willing to voice the clear and rational opinion that the lottery is an inhumane exercise in pointless brutality. Old Man Warner dismisses the notion of discarding the lottery as preposterous. "'There's always been a lottery,' he added petulantly" (215). Even the young children, who are ordinarily exempt from Jackson's critical eye of suburbia and society at large, cheerfully attend the lottery and take part in the stoning of Tess Hutchinson. "The children had stones already, and someone gave little Davy Hutchinson a few pebbles" (218). To the townspeople, the thought of dispensing with the tradition of the lottery is inconceivable, because they are too steeped in conformity to consider breaking tradition.