Not using Avada Kadavra in a wizard duel in which you intend to kill the other is like having a duel with guns with infinite ammunition and attempting to bludgeon the enemy with the hilt or stab them with a bayonet instead without ever taking a shot.
So, they do kinda explore this in the books. Book 4, when the kids learn the Killing Curses in Defense Against the Dark Arts, its implied that you really need to want to kill someone for the spell to work. Otherwise, it just kinda gives your victim a nose bleed.
There’s a secondary implication that Avada Kadavra is not just a simple killing curse, but a predicate for creating a Hoarcrux. In Book 6, Tom Riddle learns that you need to kill someone in order to create a Hoarcrux. And, as Harry Potter is a Hoarcrux created by accident when his mother shields him from Voldemort’s killing spell, there’s a presumed through-line that using the spell severs your own soul with every successful incantation.
So, the need to be fairly powerful, competent, and ruthless makes it more like going into a wizard duel with a blunderbuss than a Beretta. There are other spells that can kill (or immobilize in anticipation of a more traditional murder) with less effort. And - assuming the implications - no risk of shredding yourself internally to land a lethal blow.
So, they do kinda explore this in the books. Book 4, when the kids learn the Killing Curses in Defense Against the Dark Arts, its implied that you really need to want to kill someone for the spell to work. Otherwise, it just kinda gives your victim a nose bleed.
There’s a secondary implication that Avada Kadavra is not just a simple killing curse, but a predicate for creating a Hoarcrux. In Book 6, Tom Riddle learns that you need to kill someone in order to create a Hoarcrux. And, as Harry Potter is a Hoarcrux created by accident when his mother shields him from Voldemort’s killing spell, there’s a presumed through-line that using the spell severs your own soul with every successful incantation.
So, the need to be fairly powerful, competent, and ruthless makes it more like going into a wizard duel with a blunderbuss than a Beretta. There are other spells that can kill (or immobilize in anticipation of a more traditional murder) with less effort. And - assuming the implications - no risk of shredding yourself internally to land a lethal blow.