Why was mr.yewewl try to kill jem?

Answer 1

To get back at Atticus for saying that he was lying when in fact, Mr. Ewell was lying about that whole incident with Tom Robinson. Even though all the evidence in the trial was pointing to the fact that Mr. Ewell was lying, Tom Robinson was still convicted because of how the judges in the courts always voted for the white people. At the end of the trial, Mr. Ewell told Atticus that he would never forgive him for embarrasing him in court and trying to kill Jem was the only way that he could've possibly gotten back at Jem (or so he thought)

Answer 2

Mr. Yewewl tried to kill Jem because he believed that Jem had stolen his prized possession, a valuable necklace. In his anger and frustration, he attempted to harm Jem as retribution for the perceived theft.