"If once a man indulges himself in murder, very soon he comes to think little of robbing; and from robbing he comes next to drinking and Sabbath-breaking, and from that to incivility and procrastination."- Thomas de Quincey [1785-1859] wrote Confessions of an English Opium-Eater [1821] which was about his opium and alcohol addiction.
A contemporary perspective on murderous thoughts from Harvard Psychologist Steven Pinker here.
