If your idea of anarchy is worthwhile, it has to offer something better than what we have now in our real lives.
No it does not. It just has to offer something more ethical and more just. And besides, it is very easy to offer something better than what we have now.
You offer a society in which "Professionals with black clothing and silenced weapons" are killing people for animal cruelty and then seek to justify that by arguing its exactly how things work now.
I admitted it may have been too harsh, but it is not out of realm of what may be possible in future. Not because it will be legal or allowable, but because it will simply be possible.
Why not compare your "Professionals with black clothing and silenced weapons" instant execution system with the real world in which a death sentence requires a jury and years of careful examination of the facts? Your idea is certain to involve a lot more people being killed.
You live in sheltered world if you believe that is what requires death sentence. From what I hear, Texas is very happy to make death sentences with little review, and are proud of it. That may be true in Florida, too. America is very quick to give death sentence without jury to those living in middle east and north Africa. China and other totalitarian countries like Saudi Arabia and Iran are much happy to give death sentences without trials, and even democratic Russia is known to give death sentences that are not just. Also, supposedly 90% of accusations end up never going into court, and get settled because of threats of much harsher sentences. Even if not death sentence, many people in current "better" system have many years of their life stolen from them as they spend it in jails. I do not think it is very difficult to come up with better system, and at worst, we will just just as horrible system as we have now.
Any decent society will have a clear separation between the judicial act of deciding to kill someone and the executive act of killing them.
That is happy fairy tale that is not true, and maybe never was, simply because this rule is not given evenly to everyone. If state thinks you are really bad, they will just ignore this rule, and you know it.