Well,
The way I see it-
Each animal has it's own territory. Lemme add wolves as an example.
Wolves live together in a pack.
You have two alphas- Male. Female.
It's normally the Alphas that breed and rear pups.
If other wolves from another pack come into our packs territory, then they stand the chance of being killed.
Too many wolves in one area will put a stain on deer/moose/elk populations.
The attacking other wolves is to keep their territory safe.
Then again,
Chimps have been known to raid and eat baby baboons- for no "apparent" reason.
Then yet again,
Chimps are NOT strictly vegetarians. So- they need that meat to help suppliment their diets.
Your fish may see the goldfish as competition to his food source/breeding territory,etc.
Doesnt mean he/she is acting to be mean though. Nor does it mean he isnt.
Then again, I am not God- so, who's to REALLY say what goes through anythings mind.
In all of my 21 years on this earth and dealing with all sorts of animals.
I have never seen a MEAN animal. I have seen animals act on their instincts for survival- but not meaness.

I have seen Pitts that were conditioned to fight or die- but, after a loving touch and much time they calm down and loose their aggressive fear.
I DO believe an animal CAN have mental problems that makes them act crazy and mean.
I say these things based on years of experiance with both animals and humans alike.
When dealing with animals, and people too-
I find it best to stand back, study quietly and just let it go.
Learn from what you see, and compare it to what you thought you knew.
You may very well be surprised.
Ahh yes, *makes that my quote*.