You often need permission, then pay royalties, to copyright holders. That is the theft. In cases where you enjoy then distribute the copyrighted works of someone without their permission and avoid paying them the required royalties -- you have stolen money $$ from them. It's the same as eating at a restaurant then not paying. STEALING. You got a benefit you were supposed to pay for -- and you DID NOT PAY.
Stealing from others habitually is a very bad decision. You can justify it using any inner dialog you like.
The only caution is this: there are consequences to bad decisions you've made.
Here is the biggest consequence: if you purposely decide to hurt someone else (financially in this case), unless you are sociopathic/career criminal, you'll have an internal conflict about it. You will 'explain it away' so it doesn't feel as bad: "I do this because the law is unfair" or "everyone else is doing it".
YOU ARE LYING TO YOURSELF. You know darn well you shouldn't act that way. Once you start lying to yourself, you're going to green-light lying to yourself again.
For example: If asked in a job interview with the IRS "let's say we hire you and you have access to the IRS secure network, and after a week you discover that a lot of your IRS coworkers change personal data in our computer system and underpay their taxes. And you find your coworkers also download and seed torrent clouds with copyrighted material. Which activity would you find acceptable?"
YOU'D HAVE TO LIE TO THE IRS INTERVIEWER!! See? You'd have to say "I would never cheat on my taxes. And I download copyrighted material and seed with copyrighted material all the time."
YOU WOULDN'T SAY THAT!
Nope, you wouldn't get hired if you admitted to the IRS interviewer -- OR ANY OTHER COMPANY INTERVIEWING YOU FOR A JOB -- that you make illegal, unauthorized use of copyrighted works.
Why wouldn't you admit to the IRS job interviewer you seed torrents with copyrighted works without the owner's permission?
BECAUSE YOU KNOW IT'S WRONG. That's why.
RECAP: (1) you're stealing from an artist by not paying royalties on their film/music/etc. when using sites like KAT; (2) YOU'RE LYING TO YOUR FUTURE EMPLOYER in a job interview.
Slippery slope. In managing large groups of tech workers, what I discovered was this: if an employee is unethical or dishonest in one area of life, they seem to have a hard time restricting those bad decisions to only one area.