When someone threatens my life, I'm either going to call the cops, or try to kill them in self defense. Even if they're threatening someone else's life, you could still get away with killing them.
That fear and panic really got you it seems. Should we kill other people for infecting others with the seasonal flu virus as well, which also costs many lives per year? If you are endangered it is up to YOU to stay safe not the other way around. Do you know how many people die of car accidents? Should we kill all drivers just because they are potentially threatning someone elses life?