Atticus assumes that Jem killed Mr. Ewell. As a lawyer, it is his duty to claim it rather than cover it. He has to maintain the justice according to the law, despite his love to Jem. Mr. Tate says it is Boo rather than Jem who killed Mr. Ewell. As sheriff, he decides that Boo was saving other people's lives and does not need more attention. Their debate grows fiercely into a quarrel. The prosecution and the defense's roles are reversed between the two, compared to those in Tom Robinson's case. It violates the readers' fixed impression of them, and creates absurd effects.
Finally, Atticus and Mr. Tate