Well, yeah, starving fish will do a lot of weird things. Piranha don't attack large prey unless starved, but starving redtail piranha will take on things hundreds of times their size.
And yes, I should have clarified: I was throwing out ideas, not listing a bunch of things to all put together. Sunfish are predatory, not for keeping with small fish you don't want eaten. Sunfish and goldfish might get along, depending on space, cover, and sunfish species. Longears are only moderately aggressive most of the time, and during breeding season, they'll chase everything in the nest area but leave everything outside it alone. At least with the Texas ones. I watch them at the river sometimes, and they'll ignore anything that's not in or immediately next to the area they clean out for their nest. I also used to keep them in a freshwater tank, and my local garden center has a couple of them in the goldfish pond. Cool fish.
If you can get a warmouth sunfish, you could try one of those. They're sorta like a small freshwater grouper. Definitely don't put them with anything that isn't at least nearly as big as they are, they have large mouths, but they generally ignore things that aren't food.
Most game fish are illegal to catch and keep without permits, varying depending on state. However, sunfish aren't game fish in some states. If you catch them by legal means, you can keep them. Local regulations vary- definitely look it up.
That cat probably had something wrong with it. Pain and fear can make cats pretty violent. The appropriate response is to either fix the problem or (if the cat is too aggressive) euthanize it. Declawing is never appropriate- that'll just give you an aggressive cat that's in more pain and will bite instead of scratching. Cat bites are far worse than scratches, they're horribly filthy and very prone to getting infected.
Did it wind up having rabies, or anything else visibly wrong? If not, there's a disorder in dogs that makes them suddenly and randomly attack anything around them, then abruptly stop and show no signs that they knew what happened. They go from happy and playing to trying to kill things, then right back to happy. Theoretically there could be something similar with cats.