Self defense can give mitigating circumstances in some legal systems, but it’s not an ethical joker to kill whoever we self-decreed feels as representing a threat.

For example, we can go into treacherous schemes that pushes our neighbors into paths where the only obvious option they can still perceive as a way to escape our shenanigans is an attempt to kill us, and as they come with this very real intention to kill us, press the button we had prepared to trigger some mortal trap.