why does everyone seem to know what will happen to Ikemefuna

The Oracle of the Hills decrees that the boy must die. Okonkwo is a very superstitious man, even paranoid. He is warned not to have anything to do with who has been warned to have nothing to do with the Ikemefuna's death but everybody knows the fate of this boy. Okonkwo refuses to look weak and the people seem to know that he will be sacrificed for the village and Okonkwo's fierce and pathetic pride.

Ikemefina has come to the village as a peace offering; he was sent there by his tribe as a recompense for the murder of a Umuofian daughter. He is given to the tribe for sacrificial reasons, and everyone knows that he will ultimately be executed. Although he is raised by Okonkwo's family, they are all aware of what his eventual fate will be, it's only a matter of time before his death sentence will be carried out.