I just finished leading a book discussion this semester where we combined The Four Loves and Great Divorce. It was so much fun to read those two back to back and hear what people had to say!
I concur with the George Macdonald comment. People kept say, "Well, as George Macdonald said..." and I kept thinking to myself, "Well...as George Macdonald's CHARACTER said." I would love to find out what parallels if any there are between Macdonald's lines in Great divorce and what is in Macdonald's own literature.
As to the question of the Big Ghost and the Spirit of the murderer...I'm assuming you mean Matthew 5:21-22? "...anyone who is angry with his brother will be subject to judgement...."
I hadn't really thought about that, but it's a good point. Jesus in that verse seems to be saying that it isn't only what you DO, but what you consistently allow or even WILL yourself to think that changes your spiritual health/condition.
At the heart of it, Lewis seems to be agreeing. As the murderer said, the act of murder itself was a one time reaction that was based on an excess of anger (emotion). The more sever crime (spiritually) was the INTENTIONAL, CONCIOUS, choice of thinking evil thoughts towards his boss day in, day out. (sorry for the poor paraphrase, I don't have the book in front of me).
I thought this theme came up in many of the characters who got to where they were by consistently making little, but still bad, choices...exercising their will AGAINST what was good in little ways. There is more commentary on this in the case of the Apostate, and more evidence of the consequences in the mother, the wife, the man with the lizard, and the Tragedian and Dwarf. I'm pretty sure that this is a theme that winds its way through each of the ghosts stories.