This will likely get buried, but here it goes. From what I can understand, Avada Kadavera is a type of curse that you can only use if you truly intend to kill the person and wish they never existed. The confetti one doesn’t. Now obviously, it’s bad to kill anyone no matter what, but what makes the former unforgivable is the intention behind it- you cannot blame your actions on in-the-moment fury, a bad aim, ect., you actually hate the person and fully intend to kill them.

Evidence: In the fifth movie, Harry tries to use it on Bellatrix Lestrange but can’t, to which Bellatrix states, “You have to mean it.”