I'm afraid I find that kind of thing pretty sick.
There is, just possibly, an argument for getting rid of misformed livebearer or other fry is perhaps defensible as a quick way to kill them. But simply feeding goldfish to a predatory catfish -- simply for the fun of it -- is indefensible.
I know there are people who enjoy seeing "nature red in tooth and claw", but if that's so, they should play the game too. Live with the bears. Stop using pharmaceuticals. Catch their own food. Basically take their chances with nature just like those goldfish. Otherwise its simply sadism, the same brutal cruelty that drove Romans to the amphitheatres to watch lions eat people and gladiators fight to the death.
Looking at that film, it's obvious from the breaks that the goldfish were dumped into an alien habitat with a predator for many minutes, perhaps hours, before they died. They had plenty of time to be stressed and scared. Goldfish would take several minutes to die from asphyxiation, assuming that was the immediate cause of death.
Besides, cheap feeder fish are a good way to introduce parasites and bacteria, and a single species of prey animal cannot possibly provide a balanced diet. In my honest opinion, if you cannot wean a fish onto frozen food, or small invertebrates like earthworms and river shrimp, then that fish shouldn't really be in captivity.
Cheers,
Neale