Is there any spell other than Avada Kedavra that immediately kills?
(To be clear, not any spell that can wound and later kill, such as diffindo or sectumsempra.)
So does such a spell exist, and if so- What is it and why isn't it one of the unforgivable curses?
As you've pointed out, there are umpteen ways to use the effect of a violent spell to kill someone (or cleverly killing someone with a non-violent spell), but they all do so by *causing an effect other than the death of the person.*
Spells that are seen to be fatal (or described as nearly killing someone, or that cause an effect that could have been fatal if untreated) include Fiendfyre, Sectumsempra, Confringo, Diffindo and Stupefy.
– Valorum Dec 20 '20 at 11:31