It’s a fact some fish eat other fish, it’s just the way they are built, the goldfish isn’t a carnival since he is not eating his own kind any more than your guilty of being a cannibal. Techniclly you are a canibal if you eat any form of meat, your technically a mammal and you eat other dead mammals making you a cannibal in the rough sense.
Your goldfish is a fish, he may eat other smaller fish he may see as live food. If you dropped shrimp in the tank for him would you say he was a carnibal? Shrimp live in water too so if they are eaten he’s eating aquatic life.... canibalism.
Next issue, goldfish love nice taystee tropical fish. I found this out when I added a beautiful red and white fantail to my tank of cardinal tetra neons. The fish were gorgeous together and we had many days of these beauty’s swimming around... came down to feed one morning and the fat goldfish had just finished sucking down the last small cardinal tetra, the others swimming to her in a school trying to ward off goldfish from snacking. He got transferred to a bowel that day. Just like giving live Minnie’s to turtles, frogs, carp, or other larger fish, they are meant as feed er fish because they have no defense and are part of the natural food chain.
Other issue I’d most tropical fish don’t mix with goldfish and they kick off fast eating gold fish food. Don’t know why just have cleaned hundreds of dead stock out of friends tanks, once dead yes he’s an opportunist and will eat the remains... recycling because after all hrs a carp.. bottom feeder but pretty, they keep tanks clean but over eat and poop too much and cloud the tank like heck.