Zoosadists are defensive creatures, they instinctively know they have things to hide, and will invoke anything at their disposal to justify what they do. If they cannot justify what they are doing, they will demand you alter your moral framework so as to no longer see things in objective terms.
There was a notorious child murderer in England called Ian Brady. You may have seen youtube documentaries about him. His crimes are considered some of the worst in human history, yet when viewed in relativistic terms, how zoosadists prefer to see things...he's not really that bad a guy. Ian Brady wrote a book called Gates of Janus, and every time I see a zoosadist defend their inclinations and become angered at the majority of humanity, I'm reminded of the rheotoric employed by Brady in his true crime/philosophy book. It's a three layered approach;
Point to another bad thing, and accuse your critic of not caring about that thing.
Demand the critic only sees the crime from the moral framework of the criminal.
Project that the critic is actually the bad one, because by focusing on an individual zoosadist/child killer/rapist they are ignoring crimes that are even greater in relation to their own.
Ian Brady argued something along the lines of; politicians kill millions and nobody cares, if I wasn't born into a predatory, chaotic universe which compels us towards the survival of the fittest, or society catered for the way sadists are, we'd all be better off. The purpose of this is not necessarily to justify his own crimes, but to change the subject. When you call into question the moral standing of somebody who enjoys watching small animals being brutally tortured for prolonged periods of time, you can bet they'll want to talk about anything else. Monkey on monkey violence, how it isn't sexual, how they love cats, how 'Karens' aren't doing enough to stop CSAM, they have an abundance of talking points that are intended to make people who are appalled by them second guess themselves.
Even if just for a moment, a man who rapes and strangles children to death can plant in your mind that maybe you should hear him out, he's won a small victory. Zoosadists are no different. Their moral relativism is not intended to ease their own conscience, sadists are self preserving and lack empathy. Their moral relativism is meant to discourage people who want them exposed and held to account. It is a distraction, and a struggle to control how we perceive them.
The best response to the zoosadists moral relativism is an unwavering objectivism. Downloading, distributing, encouraging and seeking pleasure from the torture of animals is objectively wrong. We do not care about your Brady-esque Nietzchean mental gymnastics. We're not here to explore philosophy with you, or the equivalency of your crimes with others, we're here to tell your loved ones that you like to watch small animals being tortured and would escalate beyond this given the means. We know that people share videos of cartel killings on 4chan, as it stands murder is illegal and people better equipped than us are at least trying to respond to cartel violence. For now we are the only ones dealing this thoroughly with zoosadists. I wish that were not the case but it is.
You're the weird one, Karen!
What is the itch? I have heard zoosadists describe the time they spend watching animals being tortured, or torturing animals as being like 'scratching an itch'. I've heard Yardfish talk about how zoosadist videos, especially involving baby monkeys, recreate scenes from hurtcore CSAM. Despite this, zoosadists are vehement about the 'itch' they experience not being sexual, it's a compulsion, it's the way they're wired, it's morally abhorrent...but it's not sexual. This is a point of obsession for zoosadists, especially zoosadists who torture monkeys. They're as obsessed with talking about how zoosadism is not sexual as they are baby monkeys being breastfed or wanting to be held.
We've all seen crime documentaries about sadists who can only achieve 'release' in the moment their victim is dying. I imagine the itch to be something like this, something that cannot be satisfied by mundane sexual means alone. Something that runs deep into their make-up, and whilst likely well hidden, should be a monumental red flag to anybody unfortunate enough to be around that person.
That being said, a zoosadist would rather debate the finer points of the origin of their compulsion than deal with its objectively loathsome nature head on. Imagine a flasher, who upon arrested for indecent exposure has a sob story about how his brain has an itch, he feels lonely and isolated and indecent exposure allows him to feel connected to people. He'd argue that people like him exist and he's just a product of nature, that he never meant any harm.