This is the misdirection trope, where we're meant to suspect Salome for some time before new information clears her. Here's why I doubt she's the perpetrator.
If Salome's power unintentionally causes mania in men, that could motivate an infatuated Knight to take a shot through the arrow slot in the wall. What's missing is the inside collaborator to guide the knight and the saint candidate to the arrow slot at the right time when there are no witnesses. It doesn't seem like Salome intentionally wants to harm the other candidates.
That means there's likely another perpetrator aware of Salome's influence on the knights, who guided them to the arrow slot and got them to make the kill.