You are not alone, certainly you are in good company (many people get puzzled with that one).http://www.physics.ohio-state.edu/~eric ... ar%20I.pdf
7. Noun Strings
In English, an adjective is supposed to modify a noun. But there are not enough adjectives
to go around so a noun is often used in its place. Don’t overuse this construction since
meaning can be obscured; often a prepositional phrase is better.
Example: “woman killer” can mean a killer of women or a female killer
“nutrition facts” Here the meaning is clear but use of an adjective or a phrase
sounds more like English: nutritional facts, facts about nutrition
“customer service representative” If you really need such a rank construction, you
should put a hyphen between “customer” and “service” since the words used
together are a noun, here masquerading as an adjective.
Well, it goes with context. In this special case it´s already a fixed expression of sorts (an urban legend) and then by the infamous clown that was a killer (murderer), John Wayne Gacy.http://en.wikipedia.org/wiki/John_Wayne_Gacy
In fact, "the clown killer" could be misunderstood as a guy that kills clowns. As you see in the above explanation.
And "clown killer" as a random way to refer to any "clown killer" (as per above definition).