If the Killing Curse has an indefinite lifespan and ends only upon severing a soul, there may be accidental casualties of humans on the other side of the earth or perhaps alien life. Perhaps a hive mind would die instantly to a Killing Curse.
I wouldn't expect many accidental deaths. Think about how often people shoot off guns in crowded urban areas and no one is hit. The Killing Curse is cast very rarely; there are no recreational curse ranges you go out to. The other side of the earth is millions upon millions of square kilometers; humans take up much less than a meter each. So... For aliens, it would be astronomically more so: probably not a single Killing Curse would ever come anywhere near an inhabited solar system, much less hit someone there on a planet. (That would happen only if driven by plot necessity, which famously sneers at mere probability.)
This is entirely true and my original comment only stated there would be "many" deaths because of poor word choice from my sleep deprived self.