Obierika helps destroy Okonkwo's compound because it is dictated in the tribe's traditions that this is what must be done. Okonkwo has committed a crime against the earth goddess by killing one of his clansmen, the son of Ezeudu. Although the killing was inadvertent - Okonkwo's gun had gone off accidentally and the bullet had lodged in the boy's heart - the punishment is clear. Okonkwo and his family must leave the tribe, and live in exile for seven years.
As Okonkwo's friend, Obierika comes to console the distraught man, and to help him pack his things. After Okonkwo is gone, Obierika dresses in the garbs of war with the other men as tradition prescribes, and sets fire to Obierika's living quarters. Obierika and the others harbor no ill-will towards Okonkwo -
"They (have no hatred in their hearts...it (is) the justice of the earth goddess, and they (are) merely her messengers."
The bonds of tradition are strong; her customs must be obeyed, lest disaster fall upon the entire clan. Custom establishes order in the life of the tribesmen, and so it is followed faithfully, and generally without question. Obierika, however, is a thinking man, and although he does what tradition dictates, he wonders why a man should suffer so for something he did unintentionally, but although he ruminates upon the matter for a long time, he finds no answers (Chapter 13).