The humor in this meme arises from the visual pun created by part of the sign for 'PHARMACIST' being obscured, leaving the word 'HARMACIST,' which juxtaposes the role of a pharmacist with the concept of harm. This is found funny because pharmacists are typically associated with healthcare and helping people, so the idea of a pharmacist 'turning to the dark side' is an unexpected and absurd twist, referencing the 'dark side' as a metaphor for malevolence, often used in popular culture.