Much of Arrow of God's plot is precipitated by revenge. If Umuaro hadn't wanted to claim ownership of that land, they wouldn't have sent an emissary to Okperi who was clearly bent on starting a war. That emissary causes his own death, but Okperi fails to send a courteous message about it, so Umuaro must respond by starting the war. Just as entire regions seek revenge, individuals seek satisfaction for real or perceived wrongs. Ezeulu seeks revenge on the people of Umuaro, who fail to give him proper respect as the priest of Ulu. Ezeulu's revenge results in famine and ultimately causes the demise of his own deity.
Although Ezeulu believes he is following Ulu's will by punishing the people of Umuaro, it is really just a selfish attempt to enact revenge for their disrespect.
Although the people of Umuaro believe that Ezeulu is avenging himself because of his pride, Ezeulu is like any priest – he must follow the rituals and rules of his religion.