I think I like your image of two war veterans much better than my own speculations-it seems to fit! Have you read 'The Mere Wife' by Maria Dahvana Headley? It's a reimagining of Beowulf set in 21st century America that recasts the mother of Grendel as a battle hardened war veteran. It calls the nature of monstrosity into question. Seems like it could be up your lane...

The question about the murderer's beliefs is a fascinating one. Shakespeare's world-both the fictional and real one-was suffused by religious conflict but it's hard to pin down individual characters according to religious sentiment. It's easy in cases like 'King Lear' where Edmund practically declares himself an atheist but 'Macbeth'? That's another can of worms entirely.

But the fact that we don't know is probably what makes it so fascinating.