Argthtjtr

A gun can bring destruction but at the same time save lives, both curse and magic belongs to different system and operates independently.

Curse requires strong feeling such as suffering pain while magic usually needs tranquility and serenity, however both draws energy from a common source which permeate throughout the universe.

Magic can be used to bring harm when misused or even abused, but curse regardless of modus operandi is deemed as an act of cruelty and often frowned upon by the community. If the only distinction is the state of emotion while the spell took form, why only curse is forbidden? A good analogy is matter and it's anti-matter, they are very similar but have opposite charge, simply throwing a tantrum cannot manifest curse unless it resonate together with the frequency of the target.

When you are angry you can only "curse" and therefore only bring destruction - you are angry after all. when you are relaxed you can do anything because you can rationally think about your options and your goals.

"Curse" is therefore simply a name for "casting magic while in a negative state of mind" and as a negative state of mind leads to destructive magic this is deemed illegal. It's just semantics according to your definitions, therefore only "curses" are forbidden because that is equivalent to saying "you were out of control while using a very, very powerful tool". It's like drunk driving or something similarly reckless. If you accidentally harm someone with normal use of magic it's just that - an accident. You didn't plan to do something dangerous.

I would argue that the opposite of a curse is really a blessing. A curse is always intended to do harm to the target, but a blessing is always intended to benefit the target. The caster of a curse would not necessarily have to be angry. Jealousy or greed would work just as well as a motive to place a curse on somebody. Black magic may come from a dark or bad place, but could potentially be used for good. All curses may be black magic, but not all black magic is a curse. There are also other scenarios like granting immortality, which depending on your point of view can be a blessing or a curse.

You can't fuel your magic with negative emotions without it changing you as well.

So, the Unforgivable Curses in Harry Potter follow this kind of logic. After Harry casts a Cruciatus Curse fueled by righteous anger, it has a neglegible effect on Bellatrix Lestrange.

‘Never used an Unforgivable Curse before, have you, boy?’ she yelled. She had abandoned her baby voice now. ‘You need to mean them, Potter! You need to really want to cause pain – to enjoy it - righteous anger won’t hurt me for long – I’ll show you how it is done, shall I? I’ll give you a lesson –’

The key here is that it's not just anger that drives the curse, but the cruelty as well. That level of hatred and cruelty inevitably corrupt the person trying to harness it. If you don't truly feel it, you can't harness it, but if you truly feel it, it begins to take hold of your life until it consumes you.

This is also why the Sith form Star Wars appear to grow so powerful so quickly.

Luke: Vader... Is the dark side stronger?

Yoda: No, no, no. Quicker, easier, more seductive.

Because the use of curse magic reinforces the cruelty of the user, it becomes easier and easier to do, making the user very powerful but also sealing them off from the good things in the world.

It's pretty easy to imagine why a society would be against that. You can only handle so many dark wizards before you start taking steps to prevent them from being created.

It's the same logic our society uses with dogs. If a dog bites someone, more often than not it's euthanized, not because of the bite itself, most are superficial damage at best. It's because when the dog finds itself in a similar situation again the dog realizes that option worked before, so it's clearly on the table.

I deny the hypothesis.

Sometimes a curse is the only resort for someone who is tormented by another, and who cannot get justice in any other way. The deck is stacked against them, the laws favor the powerful, and the only way they can get justice is by the curse.

This, of course, is well known by those in power. It is not in the interests of the powerful to let just anyone cast a curse. Of course curses are frowned on by the community of those in power. "Look at the scum who use them!", they say to each other.

Thus the prohibition. If those in power allowed just anyone to cast a curse, they would have a populist revolution on their hands in no time.

Propaganda

Why do you think it's called something with such a strong connotation as "Curse" in the first place?

Magic is tranquility and serenity like you mention. But more than that, Magic is Order, Control and a steady strength.

"Curse," as the propagandists would have you believe, is suffering and pain. But deeper than that, it's Chaos and Change. Do you really need strong painful emotions when casting? Or just strong emotions in general? Just how much of what you know of "Curse" is actually true?

So of course the goal of those who firmly believe in Magic, Order and Control is to outlaw anything that could shake their rule. And what could be more disrupting to them than the force of disruption itself? Chaos and Change are naturally their most dangerous enemy. So how do you prevent the populous from practicing this? Outlaw it. Make it taboo. Paint it as a sick and evil art.

Of course, as any historian knows, you can't hold back Change forever. The longer you try, the more Chaos it will bring when it finally comes...

I'm sure this comparison has been drawn already, but this notion is very similar to the Light and Dark Sides of The Force. They are both sides of the same power that governs and intertwines among all living things, but the aspects of The Force that are seen as "The Dark Side" are heavily discouraged and, past a certain point, criminalized.

The difference is that the Light Side is the side that is attributed to peace, order, and serenity, whereas the Dark Side is attributed to passion and chaos. While not inherently so, the Dark Side was seen as evil because of the penchant of its followers to sow destruction. In addition, in the words of then Supreme Chancellor Palpatine, "The Dark Side of the force is a pathway to many abilities, some considered to be unnatural", which further deepened the mistrust directed toward Dark Side practitioners.

Your "curses" are similar in that they differ from "good magic" only in the nature of their source - passion as opposed to peace. That by itself isn't enough to label curses as evil, but consider the types of people who would be drawn to a more passionate form of magic. Those types are more likely to cause chaos and destruction, either accidentally as they experiment and explore, or deliberately as they seek higher forms of power. Practitioners of "good magic" would be seen as calm and noble by comparison, so they would become the standard by which all users of magic are judged.

As such, there would grow to be a historical mistrust toward users of curses that penetrates deep into people's prejudices to the extent that even suggesting their use could be seen as a criminal offense.