If no one had ever noticed anything strange or unusual about the murderer before this incident, that would certainly be very odd indeed.
True enough. After reflection - I would agree. There is usually some clue, even a subtle one. So either nobody was paying attention, or nobody was "reading the signs aright"!
If the person in question was in a cult, it's certainly possible for them to keep a low profile. And even someone suffering from very bizarre schizoid psychotic delusions may appear "fine" on the outside to most people. But in general, you don't think of a perfectly "normal" person just all of the sudden snapping and murdering a priest for Satan. Of course, we are now living in spiritually bizarre times. Time for much prayer and vigilance.