How is the Bob Ewell case ironic?
This case to me was ironic because a black man felt sorry for a white woman. In doing so, he sealed his fate because of his kindness and compassion. If Mayella's father had felt these feelings for his daughter, he would have made the children help around the house. Then she wouldn't have to invite a black man over to help her do chores more suited for a man.
The irony you are probably referring to is his being left handed. Mayella was beaten about the right side of her face. This would happen if a left handed person was punching her. Tom Robinson had a withered, crippled left arm. Therefore, he could not have inflicted the facial bruising Mayella exhibited. This point should have freed Tom Robinson. But. when Tom said he felt sorry for Mayella, that statement convicted him, not the evidence.
There is at least one other major irony concerning Bob Ewell and the trial of Tom Robinson in To Kill a Mockingbird. Bob Ewell is actually the man who attacks and beats his daughter. He is not charged, however; he, instead accuses Tom, knowing that the sheriff will take his word and that Mayella--being afraid of her father--will agree to back his story. According to Atticus, "the Ewells had been the disgrace of Maycomb for three generations." Yet his word is accepted as fact despite the evidence Atticus presents in support of Tom's innocence.