“For if once a man indulges himself in murder, very soon he comes to think little of robbing;”
De Quincey parodies moral slippery-slope arguments by absurdly treating murder as the first step toward increasingly ordinary vices such as robbery, drinking and procrastination.
Source details
Second Paper on Murder (1839), represented in Project Gutenberg #10708 under the Murder Considered as One of the Fine Arts sequence. The passage is deliberate black comedy and satire; it does not endorse violence or present a serious behavioral progression.