Question Everything
I assume McB was lying when he said he was a Quaker, it was just a prudent thing to say at the time.
It makes me wonder if he was even a Yankee soldier at all. Maybe he was just a hoodlum on the run. We have no way of knowing! He could have gotten the uniform off a dead soldier, even one he killed himself! We don't know anything.
Another motive for killing McB was Martha's possible gay feelings for Edwina. It is pretty obvious she had an incestuous relationship with her brother. McB reminded her of him?