Wiktionary asserts the etymon of reus as rēs. But how might have 'rēs' shifted semantically to mean 'reus'?
My conjecture: If I ignore the morality and subjectivity of something 'guilty', then one can consider something guilty as a thing, matter, or affair; but this conjecture is outstretching the Semantic Field of 'reus' and feels too feigned.