Earlier quoted context omitted.
Flies fly on shit and bring that in the house. That is reason for me to not have mercy. They bring harm. Wasps do not bring harm to me, I know how to coexist with them, but if my kids are around, I will also kill them if they seem aggressive. So my ethics are very pragmatic.
Killing something because it mistakenly wandered into your periphery isn't exactly what I'd call pragmatic. If our solar system drifts too close to another civilization I don't want to get squashed. That's the maxim. We are the fly. If you don't want something bigger than you to be entirely justified in killing you for no reason, you cannot believe yourself to be entirely justified doing the same. The golden rule is…
For the reason of it being a danger. And that my energy is spend more effective on other things, than trying to save life that potentially carries diseases and goes straight to any open wound or eyes. So they come on purpose, not by mistake.
If that is no reason to you, you may continue to catch and save flies if that is your thing, but please don't say "no reason" when you mean "no valid reason to you".