Jem is sad and angry when the verdict is read in To Kill a Mockingbird. He does not understand how such a miscarriage of justice could occur.
While the verdict is read, Jem is intense.
I peeked at Jem: his hands were white from gripping the balcony rail, and his shoulders jerked as if each "guilty" was a separate stab between them. (ch 22)
Old enough to understand the trial, he was sure he had followed all of the evidence and there was no way the jury could return a guilty verdict. Jem was convinced that Tom Robinson would be acquitted once Atticus established that the crime he was accused of never took place, and Tom could not physically commit it if it did.
It was Jem's turn to cry. His face was streaked with angry tears as we made our way through the cheerful crowd. "It ain't right," he muttered… (ch 22)
Atticus comments that the verdict was “a little too strong for him” because he was highly affected by the trial. He wanted to be a lawyer, and expected Atticus to win. To Jem, there is no way the jury could have found Tom guilty. Jem is in a state of shock.
Jem is at an age when fairness is important. He is also learning how the world really works. Yes, the verdict should have gone the other way. It didn't, because of the racism in Maycomb.