A saboteur is a person who makes a mess of a situation on purpose. You might call your little brother a saboteur for letting the air out of your bicycle tires, but you could be a saboteur in return by filling his shoes with cold spaghetti.
Saboteur is a noun that is fairly new to the English language; it was first used in the early 1900s, and it refers to a person who deliberately destroys or obstructs something. It comes from the French word, saboter, which really and truly means to kick something with an old-fashioned wooden shoe. We can only hope that one day the word Nikeur might enter the English language to mean a person who kicks something with a sneaker.