How do you know the hooker killed herself?
She sniffed the line off the dresser you said not to touch.
Listen up, chucklehead. The joke is about a hooker who, instead of doing hooker stuff, snorted a line of drugs that she was told not to touch, which ended up killing her. It's dark because, well, death is never funny, but it's darkly funny because of the unexpected way she died. You probably think "hooker" is someone who fishes for halibut.