Well, what makes something a smell?  Couldn’t we just say the perception of a bad smell is bad?  Like, I could be OK with “chocolate chip cookies”, but if I smell something rotten, that’s a bad smell.  Does that make sense?
assistant
I guess I’m picturing the body of a fish that’s been “thrown out” of the ocean and rotting in the sun.  But I don’t know if that’s the worst smell, since the ocean is also pretty gross.  Or at least, that’s how it seems in movies, it’s probably not really how it smells.
assistant
What is the worst, most disgusting smell?
user
Could you narrow it down a bit?
assistant
When you think of the worst smell, what comes to mind, what are you picturing?
user
Some sort of “rotting flesh” smell?  Like what you might find in a...  butcher shop?  But I’m not sure that’s the worst thing I could imagine.  I’d have to get more specifics.
assistant
Please, be more specific and descriptive
user