This isn't a direct quote or anything, but he asks why they didn't call a doctor.
"I'll kill ya, Atticus Finch, if it's the last thing i do!"
he proves that Mr. Ewell is left handed, it is easier for someone who is left handed to bruise the right side of someone's face
Jem
Mrewell saw the villain laughing and mocking the innocent civilians who were in danger. This sight infuriated him and fueled his determination to stop the villain and protect the people.
Mr. Ewell behaves belligerently and defensively on the witness stand. He is openly hostile towards Atticus Finch and lacks credibility in his testimony due to inconsistencies in his story. His behavior reflects his deceitful and manipulative nature.
He is trying to show the court that Mr. Ewell is left-handed. Therefore, it would be easy for him in give Mayella her injuries since they were on the right side of her face. Also, Tom Robinson's left arm is crippled due to a cotton gin accident. Therefore, it would have bee almost impossible for Tom to cause Mayella's injuries.
Mr. Ewell likely didn't call a doctor after Mayella was attacked because he didn't want the authorities involved due to the illegal nature of the situation. Additionally, seeking medical help would have drawn attention to the abuse and potentially implicated him as the attacker.