‘Then they spat in his face and hit him, and some smote him with the palms of their hands, saying, “Prophesy to us, you Messiah. Who is he who struck you?” '
Having obtained the verdict that they needed Jesus was first openly repudiated by symbolic actions (spitting was an acknowledged way of showing legally based contempt - compare Deuteronomy 25:9) and then handed over to the guards for horseplay. It is quite likely that members of the Sanhedrin initially took part. It was an official and open way of indicating legally based contempt for the accused. They would indeed feel it necessary to indicate their view of this man openly, and no doubt considered that by spitting on Him, slapping Him and mocking His ability to prophesy, they were doing precisely that (compare Deuteronomy 25:9 which emphasises how important physical acts of repudiation were seen to be). In those days even high level people expressed their contempt more openly and physically than they do now, and that was what they were doing here. Being able to identify those who slapped him was, according to some traditions, in line with what the Messiah was expected to be able to do. So such an idea probably made them feel justified in behaviour that disgusts us (including many modern Jews). Then He would be left in the hands of the guards who would simply imitate their betters.
The guards then also proceeded to spit in His face, and knock Him around, aping their betters, while others continued the idea of slapping Him and crying out, ‘Come on, you Messiah, prophesy who it was who hit you.' To have a supposed Messiah and prophet at their mercy was too good an opportunity to miss, and they were after all only following the example of their superiors, even if a little more brutally. Convicted prisoners were looked on as fair game. Their treatment of Him would probably be good for a few drinks among their fellow-guards as they recounted the experience afterwards. Little were they aware that they were fulfilling prophecy (Isaiah 50:6) and that they would go down in history for it.