I never said dropping a ketchup bottle was an American idiom. I said eating ketchup with fries is an American idiom. I said dropping a glass of water (or anything really) is a stock, old-school, slapstick gag. I'm sure plenty of people around the world, including Americans, are familiar with that.
This wasn't a glass of water; and vaudeville is dead. Most of the people who saw that image knew perfectly well what it implied. And again, there's absolutely no context in the song for the schtick you're describing.
Besides, blood splatter patterns have unique characteristics because of the consistency of blood, and the fact that it is under pressure inside the body. Splattered blood doesn't just come out looking like a blob. It typically includes directional patters. Of course now someone is sure to come up with a picture of a blood stain that looks just like the ketchup to "prove" me wrong.
I doubt anyone needs to prove anything. Everyone here seems to get it.
Of course, it
would be a bit more soothing to just let this thread devolve into an international discussion about condiments.