system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Why are there only 3 unforgivable curses?

I feel like there's a bunch of other terrible spells yet they're not made illegal by the ministry. 

Like sectumsempra for example. Is it because the Ministry doesn't know this spell exist since Snape created it not that long ago when he was a student?

What about obliviate and whatever spell Voldemort used to create his horcruxes? Those 2 spells can be quite dangerous in the wrong hands.

Also, if a spell like Avada Kedavra is illegal and cause instant death, how do wizards even practice it? Like it's kind of obvious of Voldemort practiced it but what about the good wizards like Moody, how did he practice that spell without killing anyone?
gpt
Most spells have some utility. Obliviate is necessary to keeping the wizarding world secret. Sectumsempra is like a knife or a chainsaw-- it can cut many things, including people, but not only people.

The Unforgivable Curses stand apart because their only functions are to hurt, kill, or enslave. They're not useful for anything but violence.

Horcruxes aren't created by a spell, there's normally a whole complicated ritual that involves doing something unspeakably profane to the murder victim. It probably *is* illegal, but since it's not a spell it's not an Unforgivable Curse.

Moody probably doesn't practice it, remember that by the time he got the teaching job he had already been kidnapped and replaced by Barty Crouch Jr., who was a death eater and had probably used it before. That said, the implication seems to be that they are not so much difficult in terms of being complicated as they are in terms of requiring a lot of effort and intent. The way you practice it is by becoming more comfortable with the idea of hating someone to death.