Who has seen "In Bruges"?

I rented "In Bruges" a few months ago and recommended it to a friend. He told me yesterday that he just rented and did like it; however, he had a question. My memory is so bad I couldn't give him an answer. The question is: Why was the priest targetted to be assassinated in the first place? I learned the reason when I saw the film, but I just can't remember now.....