What crime does Okonkwo commit during the proceedings, and what punishment does he recieve? Why is it necessary?

During Ezeudu's funeral, Okonkwo's gun explodes suddenly. A piece of iron pierces the heart of one of Ezeudu's sons. Even though the death is accidental, the act is an abomination to the Igbo. Okonkwo is to be exiled for seven years.