Well, considering how many people slow down at a car accident to get 'a good look', I'd say it's the majority of people who enjoy looking at carnage, rather than the minority.
I feed my fish live bloodworms and grindal worms. I could feed them microworms, but I figure that would be only half the fun for them. I don't know about you, but I like to chew my food, rather than swallow teeny bite sized blocks.
Just like you said, where do you draw the line? Is feeding a goldfish too much? Is feeding a live bloodworm, trying to get away from my dwarf puffer too much? I watch them eat, and I enjoy it every time. Not because I like to see the bloodworm suffer, but because I get to watch my fish enjoy a meal.
Personally, I cringe everytime someone writes something like this here: "I like my guppies, their fry feed all my fish!", but I'd never think of bagging on them because of that. It's their choice.
I'm old enough to remember the Summer Olympics in Seoul, South Korea. I also remember a couple newspapers printing pre-made complaints that we were supposed to fill out and send to the South Korean Embassy and Olympic Committee, demanding that the games be held in another country, due to Koreans eating dogs. Personally, I'm don't eat dog, but who am I to tell another culture what to eat? Yes, they surely suffer from what I've seen, but then again, I have no clue what the cow went through that's responsible for my double cheeseburger with chili that I had yesterday. And I don't see a single Korean writing in to the state department, demanding to ban cow eating in my country.
To derail this further, unless everyone offended by this turns vegan immediately, I'd say they're hippocrites.
Now, I do agree, that the people who filmed this don't seem mature, but it's not because they're feeding a goldfish (of any size) to their piranhas.