If he wants to attack, he has to do so before the opponent reacts.
Benjamin understood this principle very well.
In this situation, he could also turn and run. He could let these Priests not find a soul, but what meaning would that have? On one hand, the three priests were about to realise the existence of a spy. Their wariness would increase tremendously, to the point that it would greatly affect the Church’s secret plans in Icor.
But on the other, Benjamin still very much wanted to clarify what exactly the Church’s plans were.
Furthermore, from the way the Priests attacked and killed the pig, their skills were probably not that high.