system
A chat on subreddit /r/NoStupidQuestions
human
There's an unofficial scale, from "least like a human" to "actually human" for all life forms. Stepping on an ant isn't an issue for anyone except some very devout monks. Running around shooting humans for fun is generally frowned upon by anyone except a handful of psychotic serial killers.  
The question of how far along that scale is considered acceptable is different for everyone, but it's a general bell curve of the majority opinion, with fish being closer to the left side, nearer to smacking spiders with a newspaper or putting down roach poison, and hunting deer over to the right side - past setting out rat traps, and beyond squirrel hunting. Even further to the right might be bear hunting, and poaching chimpanzees is probably the closest thing next to homicide.  
It's also why hunting dolphins or whales is so much worse than fishing for trout or lobster. We might not have  a clearly defined taxonomy, but we are innately aware that a warm blooded dolphin that communicates with family,  gives milk and nurtures its young is closer to us than a haddock.
gpt
An excellent, well-worded and comprehensive reply.  Thank you.