The Cruelty isn’t in one fish eating another. It’s in placing two incompatible fish together in a way that’s going to end badly for simple shock/amusement value.
The tank is too small for either of the two large fish alone. Moving a large fish to a small tank simply to feed it is stressful to the fish and can result in injury or stress related infection.
This isn’t something “nature would throw down” In nature those fish don’t coexist. One is Asian, the other South American. If they did, they would inhabit completely different portions of the water column.
If you are going to feed the catfish feeder fish rather than the readily available pellet food, they should be sized appropriately so that the catfish can swallow them, and the baitfish will die quickly.
For the record I ran a pet store. I’ve got some not so amusing stories about folks who tried to feed live rodents to snakes that were too small…