What does Guitar mean when he says "everybody wants the life of a black man"? Do you think he is justified in joining the Seven Days?
I can comment on part of your question concerning Song of Solomon. Your question about Guitar's joining the Seven Days is largely a matter of opinion. I can give you my answer, but I can't give you yours.
I can break the issue down for you a bit, however.
It seems to me that the issue has two sides: the emotional and the rational. In the context of the book, emotionally, I cheer (figuratively, of course) when I read about the secret group that avenges senseless murder. It's about time! It's easy to applaud this correction of injustice.
Rationally, however, one knows this is not the answer. In a broad sense, the eye for an eye seems satisfying. Specifically, though, it's just stupid. It doesn't accomplish anything and it corrects nothing. It just takes one senseless murder and doubles it.
And, of course, mistakes are made, as the conclusion of the work demonstrates: the innocent are bound to suffer with the guilty.
I'm afraid I don't have a copy of the book with me at the moment, so I can't help you with the other part of your question. I need to refresh my memory of the context of the quote, and I don't have any way of doing that.