Principle of double effect
Historians think Thomas Aquinas was the first person to write about the principle of double effect but they are not sure.  Thomas Aquinas lived in the 1200s.  He wrote about killing in self-defense.  Philosophers from before Thomas Aquinas, for example Augustine of Hippo, said it was never acceptable to kill another person, not even in self-defense.  Thomas Aquinas said it was so long as the person did not kill their attacker on purpose and so long as the person does not fight harder than they need to.  Here, the first effect is saving one's own life.  The double effect (side effect) is killing the attacker.  He wrote this in Summa Theologica.