Dom—Bento—wants to find some kind of absolution through telling his story to the reader. He believes that if he can convince a reader that his suspicions were correct, he'll be able to convince himself of the same. He approaches his story like a lawyer speaking to a courtroom full of jurors, laying out the supposed evidence and trying to convince them to listen and believe him.
It isn't easy for Bento to marry his love, Capitu. His mother believed that he'd be a priest, and that was the life he was building. His best friend, Escobar, does a lot to help them be together. It seems like Bento is going to have an excellent life—until his son is born and resembles Escobar more than Bento.
This realization and the belief that Capitu and Escobar have had an affair poisons Bento's life. It's too late to ask Escobar for the truth, because he's drowned. Capitu and Bento divorce, and she leaves for Europe with their son; he never sees her again. The family members that Bento has left die, Capitu dies without seeing him again, and even their son dies before Bento.
In the end, Bento is alone and wants to believe that it isn't through his own actions. By telling the story to the reader, he hopes to be judged and found innocent of any wrongdoing. Perhaps he also hopes to find himself innocent in the process.