system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it considered fine to kill insects for little to no reason but not say, raccoons or birds?
gpt
If you're looking for a single, objective, consistently applied criterion for ethical justification of killing animals, you won't find it here (or anywhere else).  The value that people place on different types of animals is largely subjective, and also differs from one culture to another.  For example, many followers of Jainism and Buddhism would say that killing insects is absolutely a wrong thing to do.

But for peoples that do place different values on different animals, criteria often used (either explicitly or implicitly) to assign value to animal lives might include (in no particular order):

* Ability to feel pain
* Mental capacity
* Emotional capacity
* Similarity to humans
* Value to humans
* Danger to humans

Keep in mind that while some of these criteria (specifically, regarding relationships with humans) might be quantifiable, others are based on perception and can be quite subjective.