Is the 'Avada Kedavra' Killing Curse Broken?
Halfway through the Harry Potter series, the transformative franchise for the majority of millennials, we were introduced to the unforgivable curses. The trio is comprised of the torturing Cruciatus Curse, the mind-controlling Imperius curse, and the most significant death-inducing Killing Curse. The incantation for the latter, Avada Kedavra, is derived from Aramaic (a Syrian dialect of a Semitic language) and is the original for Abracadabra which loosely translates to 'let the thing be destroyed'. Through the last four books, it's used to kill a number of named characters, including Cedric Diggory, Albus Dumbledore, Alastor Moody, Hedwig, Gellert Grindelwald, Charity Burbage, and Griphook (not to mention its many unsuccessful uses against Harry, Ron, and Hermione). But the introduction of the killing curse presented several problems for the storytelling as the saga neared its conclusion.
The Death Eaters – Lord Voldemort's sworn followers – aren't really a considerate bunch. When performing official Death Eater duties, it's unlikely that things like compassion or mercy would be required for the job. So, why would they ever use anything except the Killing Curse? Why do they even have to be good wizards? Is being a pure blood literally the only requirement for being a Death Eater? Also, how dumb of a name is 'Death Eater'? In the battle at the Department of Mysteries, many of the Death Eaters used a number of non-killing curses, including Impedimenta, Stupefy, Tarantallegra, and unnamed jets of 'red light'. They even knew that none of their opponents could (or knew how to) use the Killing Curse, and still didn't take advantage of it. Given their intention and situation, it seemed like a killing-only approach would have been the most efficient – after all, they only got one total kill (Sirius Black), and that wasn't even from the Killing Curse!
The mechanics of the Curse also favor the sides of evil a bit too much. Not only is it relatively easy to use (a short incantation), it can't be deflected or blocked by a shield charm or any defensive spells. Sure it's six syllables, but still shorter than Anteoculatia, Arania Exumai, and Arresto Momentum, so not unreasonably long. The only known counters are to use a physical object to block the blast, or to cast another attack spell (e.g. Stupefy or Expelliarmus) and hope the beams collide. It might have been valid if it had some more stringent requirements, like if it was a very focused beam that had to hit the victim directly in the heart, or if it had a cap on its range, or if the incantation was longer, or even if it required a more intricate movement of the wand (for some reason Wingardium Leviosa has the most complex motion).
In his Defense Against the Dark Arts class, Mad-Eye Moody (really Barty Crouch Jr.) specified that the user does need to have some level of magical power, and implied there was some scale to the curse's power (saying that if the class of fourth-years used the incantation against him it would barely result in a nosebleed). This can be extrapolated to other unforgivable curses, as seen in Harry's attempt to use the Cruciatus Curse against Bellatrix Lestrange following her murder of Sirius – an attempt that had some effect, but didn't utilize the full power of her spell since it was rooted in (as Bellatrix herself described it) "righteous" anger. This only handicaps good wizards further, and if someone is willing to serve as a part of Voldemort's army, there is a good chance that their magical power is sufficient (though it's worth mentioning that Harry did use the Imperius curse somewhat effortlessly on a goblin in Hallows).
Of any young adult series, Eragon has the most sophisticated magical construct. There is an Ancient Language from where the magic derives its power, so you can actually create very complex spells by saying full sentences. There's a very important plot point where Eragon, in an attempt to bless a small child, screws up a verb conjugation and says "may you be a shield from misfortune" (as opposed to shielded from misfortune as he intended). There's also a heavy use of one's surroundings, actual physical armor, and mental focus when casting the spells. While Rowling's system isn't nonsensical, it's rather simplistic, which takes away what should be the most exciting element of the wizarding world. In Eragon, there are a dozen ways to kill known collectively as the 'Twelve Words of Death', none of which require too much energy to cast (the energy cost of a spell correlates with the energy of physically doing that action). They worked with small actions like severing a vital artery or a nerve (an action that does not require much physical energy); however that also meant that they could be blocked by a defensive ward without too much cost to the potential victim. As such, they're only useful as tools of execution against a helpless opponent, enemies unable to protect themselves using magic.
One of the most intense moments of the endgame is a showdown between professors McGonagall and Snape that causes Snape to flee the castle, where the two duel with spells that directly counter each other's – a duel which was reduced to beams of light for the movie adaptation. It should have been an immediate tell that Snape didn't attempt to use the Killing Curse at all, a hint that he didn't actually want to hurt McGonagall. The nature of the Curse completely destroys the standard dueling format (the one that gave us this fight and Voldemort vs. Dumbledore), since you can just murder the person trying to shoot water at you or whatever.
It also brings up the question of why there haven't been more great dark wizards in all of history. Apart from Voldemort and Grindelwald, only a few have even been mentioned by name, like Emeric the Evil, Merwyn the Malicious, and Barnabas Deverill. But if mastery of one curse is all it really takes – and it does – then even dumbass Gregory Goyle could one day take his mantle as the next great antagonist.