Who is responsible for Rhisiart's murder in the novel, A Morbid Taste for Bones?

Columbanus is initially presented as a model worshipper, submissive to his prior and always ready to take on extra duties. There is something suspicious, however, about his frequent falls into mysterious trances where he claims to communicate with Saint Winifred. By the end of the novel, we learn that Columbanus' episodes are an act, and that he is responsible for Rhisiart's murder.