> the difference between a kid squishing an ant and a kid squishing a puppy

In western society we generally find it acceptable to kill ants and chickens and cows, but not cats or dogs or horses. But some people have ants as pets, and in some countries you eat horses or dogs while in others cows are safe from consumption.

Who defines the line between where sniffing a life is “harmless” or “serial killer shit”?

Killing doesn't violate a law of nature, it violates the social contract. So while the line is drawn differently in each society, crossing it is the same indicator.

As with obscenity, harassment, etc., there are many things that are “I know it when I see it” in society.

How many hail merry's should I say for eventually neglecting my Tamagotchi in the third grade father?

This is like squashing ants and then the ants form a borg collective and turn you into larva food.

Which is which, btw?