The phrase refers to the idea that both reptiles (in a literal sense) and humans (in a figurative sense) can be snakes. A person that is described as a "snake" would be one dishonest, devious, or treacherous. The use of "alike" here fits a certain pattern. Examples: "The snow was so heavy that it forced the closures of businesses and school alike." "Pizza at a birthday party would be popular with boys and girls alike." "English can be tricky at times for both new learners and native speakers alike." See the pattern? It's a more elegant way of saying, "Both boys and girls like pizza," or "Both schools and businesses closed because of the snow." Anyway, maybe this isn't the best explanation but hopefully it helps!