As mentioned earlier, nearly all fish are carnivorous, meaning they actively kill and eat prey. This even holds true for species such as the Siamese algae eater, a true bane of microorganisms. In addition to this, while not all carnivorous fish are piscivores (creatures that prey predominately on fish), they will happily choose another fish over many other things if given the chance. So basically, almost every cute little fish you can spot at the lfs is a "predatory fish". Your tetras will eat your guppy fry, and your piranha will eat your goldfish.
I think that when people speak of predatory fish, they mean fish that have more obvious features geared towards murdering/maiming/mauling, whether it be an overly aggressive attitude, stupid big teeth, or being covered in spines. Some times these "monstrous" looking fish are indeed psychotic murderers who can't stand the presence of another living thing, but more often it seems that they're just like other fish, they'll coexist peacefully among similar sized fish (as long as certain criteria are met), and eat smaller fish. Some species are territorial and aggressive by default, but this doesn't seem to have anything to do with outward appearance at all, and is really a seperate evolutionary adaption. Conversely, most Cichlid species are exceedingly aggressive and territorial, while I can't think of a single one that looks the least bit frightening. What I'm trying to say here is that almost all species of fish are murdering bastards, some just take it further than others.
Also a note about Bichir cannibalism, although it seems to pop up here every now and then (after all this is the internet metropolis for people who like ****ed up fish) it's probably a rare phenomenon. Polypterids sport some seriously awesome armour, their scales are several times thicker and harder than most other fish species. It really can't be properly said how incredibly strong their armor is relative to its flexibility. This means that in a community fish tank, a Bichir or Ropefish is probably pretty far down on the "who I'd like to eat" list of any given fish. So when it does happen, despite food being plentiful, it's probably because the perpetrator didn't like the victim's face or some such. A dispute. Also I really can't think of a more complete way of winning an argument than literally killing and eating the opposition.
