Tiny little fuck fish from hell, they've got teeth straight out of a horror film, and yes they've been used for it.
If Piranhas ever start actually tearing swimmers to pieces, it's going to be because they're tired of being depicted as monsters despite the fact that they're generally docile. Towards humans anyway, piranhas tend to be aggressive towards weaker members of their groups. Teddy Roosevelt encountered famous "man-eaters" during his trip through the amazon - though the particular piranhas he saw then had been captured by local fishermen and purposefully starved for days to ensure the president a good show. The locals sliced up a cow and threw it in to the ravenous fish, producing the popular "piranhas eat really fast" genre of horror movies. A school of piranhas couldn't eat that much, by the way. Even if they had been starved to the brink of death, a group of hundreds of individuals could only eat maybe 3 or 4 people before they were sated. And with the worlds obesity rates, a single individual could probably feed the whole lot.
But wait! There's more!
Seriously though, if you ever get a chance to see a move called Mega Piranhas....don't.