Addendum: A better way to define it would also have been to say that the spell effects themselves aren’t directly what leads to such harsh punishment. Instead, they carry symbolic meaning, and are the signature spells of the Death Eaters, that were appropriated by them.

So basically, there are alternative spells you could use to achieve similar effects – but by using one of these 3 particular ones, you are outing yourself as a Wizard Nazi. So basically the spell form of a swastika, wolfsangel, sonnenrad, etc.

Why is the “instant death” spell considered unforgiveable?

I mean, broadly speaking, because it murders people.

why a spell whose effect can not only be achieved by a huge number of other spells, even ones cast without speaking (e.g. the wand-flick gunshot), but which can also be achieved through basic muggle technology?

So much shit in Wizard World can just be reversed out. You can literally regrow your bones, ffs. The Big Three are politically problematic as much as they are ethically challenged. Mind Control spell created a serious problem of judicial inquiry, as any individual could plausibly claim “I was under the influence” when accused of a crime. Torture was a threat to the bureaucracy, as - similar to Mind Control - it compelled the release of classified information to outside parties. And the Murder Spell was a problem, I suspect, primarily because only Very Powerful Wizards could stick the landing. Weaker political figures didn’t want to get kicked off the ladder by some rogue asshole with magical muscles.

Why is a spell like “Sectum Sempra” considered acceptable

Its not. This is a spell that one wizard invented in his free time and virtually nobody else knows. Its not on the Big Three list by way of obscurity. Harry only avoids expulsion and a ticket to Azkabahan because his use of the spell is covered up.

A more intelligent author might have defined entire categories of spells as unforgiveable, like a wizard Geneva Convention, and would have probably included stuff like Horcruxes, necromancy, etc.

Sure. But its a children’s book not a legal text. She picks three narrative-significant spells and highlights them as Unforgiveable primarily in the negative. These are the three primary weapons of the Snake Eaters and so the liberal technocracy decided to explicitly ban their use in response to the uprising. Introducing them as “Unforgivable” gives her an opportunity to outline who used them, how they were used, and why they play such a significant role in the story.

Introducing a chapter on “The regulations and prohibitions with regard to alchemical transmutations” might make for some interesting Deep Lore, but won’t have any serious significance in a book about Wizard Middle School.

like a wizard Geneva Convention

IRL weapons control treaties are also very strange and inconsistent a lot of the time. You can burn a man alive with a flamethrower, but it’s illegal to blind him with a laser.

I guess there is no other application for an instant death curse. But a fire spell could do other things