What contrast does the author create between the banker and one of his guests

I think you are referring to the Lawyer who disagrees with the banker. The banker argues that the death penalty is more humane than imprisonment for life. The lawyer counters that, "The death sentence and the life sentence are equally immoral, but if I had to choose between the death penalty and imprisonment for life, I would certainly choose the second. To live anyhow is better than not at all."