You asked why animal abuse is considered unethical. Not why it's considered unnatural (which it isn't), not why it's considered rectifiable (which it isn't).

And no, most people don't select good and bad "arbitrarily". Most people don't just select their beliefs on morality randomly. You could suggest that individuals's emotions or reactions to things and events are subjective, but moral logic is usually based on the idea that we should try producing maximum good for all living things (including animals).

I can't really decide whether or not you don't understand the points I've laid out so far despite their clarity, or you just created this thread because you thought we'd all just take your position and let you establish your point. Either way, I've got better things to do than to convince you, but I'd like to suggest instead of trying to do this so you can convince yourself you don't need to change, you try to understand the opposition in the hopes of making yourself a better person.