Same thing as with "scarecrow". Makes no sense.
killjoy sounds better.
try putting a definite article (grammar? WTF?) between the two words and you will see that it makes more sense. For example:
killjoy Kill (the) joy. Someone who brings everyone down for one reason or another. Often applied to people in authority over us who seem to delight in denying us our fun.
If you reverse it: Joy (the) kill, well it doesn't quite work out as well.
To have the same basic meaning you would need to put the suffix -er on the end as in: Joy Killer.
Scarecrow works the same way: Scare (the) crow. If you reverse it: Crow (the) scare… it just doesn't make sense.
Here we do not have an option to use the -er suffix. Crowscarer might be a good band name but it jut doesn't work for anything else.
Hope ya learned something!
@19,048+10 internets to you. I got the scarecrow thing, but I assumed killjoy was just because it sounded cool.