In the Harry Potter universe, Avada Kedavra, the Killing Curse, is classified as "Unforgivable". The use of it on a fellow human being is enough to earn a life sentence in Azkaban.
There are other ways of killing someone with spells in Harry Potter, however. For example, when Molly Weasley is dueling Bellatrix in the final battle, it is depicted (at least in the film) that the spell Molly uses seems to cause Bellatrix to crumble away, in the same style as what happens as a result of Thanos' snap in Avengers: Infinity War). I can't imagine this has earned Molly a life sentence for using magic to kill.
Why is Avada Kevadra considered "Unforgivable" and not the other spells that are capable of killing?