“There’s no point in killing a bad priest, I’m going to kill you because you’re innocent.”
By Killing an 'innocent' priest, the Murderer seeks to denounce the Catholic Church as an institution, instead of punishing an individual for actual misconduct...
The Murder, being 'cold-blooded' will not be perceived as an Vendetta, but as an act of retribution; a twisted penance towards the church for their past sins, Justice from injustice.
The film disects the relevance Roman Catholocism constantly, as it seems every character is in some way antagonistic towards the priesthood, but tellingly none seem to hold any personal grievence with Father Lavelle, who is knowledgable of and reluctant to align himself in favor with the wider historical actions of the Catholic Church:
Michael Fitzgerald: You're a priest, are you not? A Representative for
the Catholic Church?
Father Lavelle: If you say so.
He also has a number of discussions throughout the film about the injustices of the Catholic Church, and airs grievances with their particulr handling of Paedophilic priests in his discussions with Inspector Stanton.
The fact that Lavelle hasn't actually done anything wrong is, for the murderer, the entire point.
By giving Father Lavelle 7 days to "Get his house in order", he's actually seeing if he has the 'faith' to stick around. It's a test to see if he truly possesses the noble tenants of his vocation, or if he will abandon Sligo (and with it, his own faith) in order to futher his own interests (in this case, his survival).
Lavelle must, like Christ, sacrifice himself. If he doesn't, he would be unable to live up to his responsibilites to the wider community, and the obligations of priesthood, and to come to terms with his own personal Golgotha.
But for Lavelle its more than that, its a crisis of faith on a very personal level. Sligo is a veritable viper pit, each character in some way tempting Lavelle to abandon his vocation, like the temptation of Christ.
Testing the faith of a wicked man serves no purpose, and sends no message other than revenge. If an innocent man is killed, willingly, in order to stop others being harmed, it could be perceived as a form of redemption; at least from the perspective of the Killer, who seeks to draw attention to the hypocrisy of the church.
Father Lavelle must discover if he has the capacity for the most saintly, Christlike virtue: self-sacrifice, or a noble suicide ... despite the dichotomy that concept created within Catholic values, as commented directly by the film;
Fiona Lavelle: He was an Italian Author, killed himself. Before he did he made a list of all the famous suicides in History. He included
Father James Lavelle: Sounds like a bit of a smartarse.
Press notes mention that The Guard and Calvary are the first two installments of a trilogy that will conclude with a film titled The Lame Shall Enter First, perhaps for a deeper understanding of the themes that McDonagh circulates, check out The Guard; which is much more a straightforward comedy, but no less profound.