‘And as they were seeking to kill him, news came up to the chief captain (chiliarch) of the band, that all Jerusalem was in confusion, and at once he took soldiers and centurions, and ran down on them, and they, when they saw the chief captain and the soldiers, left off beating Paul.'
‘As they were seeking to kill him.' This suggests that their intention was to beat him to death. The idea would be to ‘cleanse' the temple by the destruction of what had defiled it. Fortunately for Paul, as they began the process of mortal beatings the situation was reported to the chief captain of the auxiliaries on duty in Fortress Antonia next to the Temple. It was Roman practise to have a strong force (an auxiliary cohort of about 1,000 men including a cavalry squadron) there at all seasons when there was likely to be trouble in Jerusalem, for they were only too well aware of how easily the Jews could ‘fly off the handle'. And Pentecost was one of those times. And the court of the Gentiles where Paul now was, was visible from the fortress.
As soon as the chief captain received the report he called on his centurions and their men (there would always be some on duty ‘at the ready' for exactly such a situation) and running down the steps from the fortress they came down on the crowd. This was a well rehearsed action. It was required only too often. And the moment that the crowd saw the soldiers, they stopped beating Paul. None would want to be caught in the act and be seen as personally involved. It could so easily result in a beating for themselves, even if only as witnesses (witnesses were regularly beaten in order to ensure that they told all).