I think that people are naturally afraid of anything that is different, and that includes a fish with teeth. From our perspective, fish are not supposed to have teeth and are not supposed to be able to attack or eat humans.
I have to agree with the other posters. Since I was little, I only remember hearing horrifying things about piranhas (they could eat a whole cow in seconds, fishermen would lose limbs fishing in the Amazon, and even small children had been killed). Funny thing is, those enormous teeth I always heard about are not that enormous. Another thing, there must be blood in the water to force the feeding frenzy. Still, I would not want to get into water with one.
Piranha make great villains because they are small, voracious, vicious, and seemingly unstoppable. It's one thing to be swallowed by a whale or have large chunks taken out of you by a shark. It's in some ways all the scarier to be attacked by a horde of small fish with big teeth who travel in packs and who seem relentless. The fact that they are in rivers -- not out in the ocean -- also makes them seem in some ways very frightening.
Big teeth. Flesh-eating tendencies. Sounds the stuff of great movies to me. Of course, a piranha is not "bad," as it just does what it was made to do. The appeal of the piranha in popular entertainment is that it eats human flesh and therefore is scary
This isn't about zoology, it's about entertainment and culture. We're afraid of anything that can potentially eat us and we sometimes make movies in which we use that fear to entertain. It's like sharks and the movie "Jaws."
Anything that is scary will sometimes be portrayed as bad. It doesn't mean the fish are actually in some way evil.
Thanks all for sharing your wonderful opinion on this issue.