why was Onkowo exiled? why was it ironic?

At Ezeudu's funeral, Okonkwo's gun accidentally goes off killing Ezeudu’s sixteen-year-old son.This is a crime against the Earth Goddess and for this he is exiled for seven years. Although the event is an accident, it is ironic that Ezeudu was the man who warned Okonkwo not to take hand in Ikemefuna's death.

How did obierika justify destroying okonkwos living quarters, animals, and barn