What makes "Dark arts" dark in Harry Potter?

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 I always assumed there is some sort of hard delineation between the "ordinary" magic and the "dark arts" in the Harry Potter universe.

I always assumed there must be some special reason why the three unforgivable curses are a class of their own.

With the latest craze around Hogwarts Legacy, and all the Youtubers happily burning their enemies to ashes while slamming them up and down on the floor, only to vary it up with an occasional torture curse, I decided to look it up.

It turns out I was wrong.

"The Dark Arts,[1] also known as Dark Magic,[2][3] was a term that referred to any type of magic that was mainly used to cause harm to, exert control over, or even kill people and creatures.[4][1] Most people who practised what was known as the Dark Arts were evil, but not all."

This is the definition from the official Wiki.

Considering you can kill, torture, or even control another with an ordinary spell, the only difference between "dark magic" and the rest is that the former is specialised enough that you can't do harm by accident. I would even argue that this feature makes it safer to use.

There is no such thing as accidental misfire with dark magic. Its use is by definition premeditated and thought out.

Now for the unforgivable curses:

"The Unforgivable Curses were three of the most powerful and sinister spells known to the wizarding world,[1] as well as the strongest known Dark spells in existence.[2] They were tools of the Dark Arts and were first classified as "Unforgivable" in 1717, with the strictest penalties attached to their use.[3]"

I always thought there was more to it. Like there was deeper reasoning why those three would be unforgivable. I assumed, that the use of those curses would at least scar the soul of its caster or some such. I thought that dark magic would be powerful, but at a terrible price, that rare few would be willing to pay. 

That made sense to me. It turns out it's all just a trivial matter that somebody decided once upon a time.

What does this all mean?

That the delineation between good and evil in Harry Potter is arbitrary and trivial. What is good or allowed depends on some random sheet of paper, and can be changed at any given time. What is considered dark magic is called as such because the people who like to use it are usually wicked, or mean, or evil. But there is nothing inherently good, bad, or anything else about the dark magic spells in Harry Potter universe. They are just tools for the job.

The "good guys" are on the other hand "good" because unlike the dark wizards, they kill people with normal spells where they burn them to death. Or petrify and throw them into an ocean. I made up those examples, but according to provided definitions, people would still die, while the killers would not be considered dark wizards.

While dark wizards use specialised tools for their evil equivalent to a mace or a flail, the regular wizards are fine using the equivalent of a blacksmith's hammer. Both can achieve the same result, one is just meant for that purpose while the other is among other things also capable of it.

We can argue the intentions, try to whitewash it as a last resort, say it was in self-defence. But at the end of the day, there is no real difference between dark arts and the rest of the magic in HP universe. Because the only thing that separates dark magic in Harry Potter from the rest of it is its specialisation.

And while fire spells can be "accidentally" used for torture, for threats, and for killing, unforgivable curses cannot.