Why was Susan killed in Equalizer?

Excuse me, could you elaborate on the motivation behind Susan's tragic demise in the film "The Equalizer"? I understand that the plot involves vigilante justice and action-packed sequences, but what specific factors led to the decision to eliminate Susan's character? Was it a plot twist to create suspense, or was it a necessary step in advancing the narrative? I'm genuinely curious to know the creative reasoning behind this decision, as it seems to have had a profound impact on the overall story.