The slapping of Jesus fulfilled scripture and types (Job 16:10, Isa 50:6, Mic 5:1). This indignity also happens to those who follow Christ (Acts 23:2). However, this action was another of a long list of illegalities in the trial of Jesus.

Job 16:10  Men open their mouths to jeer at me; they strike my cheek in scorn and unite together against me.

Isaiah 50:6  I offered my back to those who beat me, my cheeks to those who pulled out my beard; I did not hide my face from mocking and spitting.

Micah 5:1  Marshal your troops, O city of troops, for a siege is laid against us. They will strike Israel’s ruler on the cheek with a rod.

Acts 23:2  At this the high priest Ananias ordered those standing near Paul to strike him on the mouth.