This is something I've been thinking a lot about recently but I'm still not sure what is right. My rule of thumb has been that I'll kill any bug that is infesting my house or causing a problem to my health. Like I would kill termites or bed bugs if I had those, or like a leech or a tick. But recently I've been getting a lot of flies in my apartment and while they are technically "infesting" the place, they're not doing anything to harm me, just annoy me. Is it wrong to kill them?
Also I would like to say that if I encountered any of these bugs out in nature I would leave them alone and not harm them, this is only in the context of a threat to your sanitation or home
Indoors? Except spiders, anything too small to put outside gets killed before it can lay eggs and create a bigger problem or spread disease. The occasional tiny spider is fine to ignore as long as they're not in a food prep area or bedroom, but it's usually a sign of a bigger problem. They tend to go away when their prey does.
Outdoors? Anything that hurts the plants.
Obviously mosquitoes and ticks get killed in any and all circumstances because diseases.