There is something all too appropriate about comparing AK to a gun.

On the other hand, there are other spells that could be equally lethal, like Diffendo (a cutting spell) or Fiendfyre, and those aren't "Unforgivable."

That 'unforgivable' label always seemed utterly arbitrary. Yes, torture, coercion and killing tend to be nasty things to do but there are far more ways to go about doing it than those three spells. Effective use of winguardium leviosa could kill dozens of people at once, for example. And combining healing magic with a sharp stick over a period of a month is probably worse than crucio for a couple of seconds. Then there's the old 'sleep/stab' combination that makes 'sleep' the most feared spell of all in certain magical worlds.

I suppose the thing about Avada Kedavra is that there's no defense against it.

That seems to be the big distinguishing feature. Teaching 12 year olds something that Dumbledore himself could not protect anyone against seems like it may have downsides.

I've always taken the position that stigmatising AK was arbitrary and pointless but I've never quite taken that position all the way to teaching junior grades how to use it. Surely it is something that should at least have the limitations that are in place for apparition? (Even if that just means removing the limits RE: apparition!)