Human Being!

Humans were my first guess, so I'll say the saltwater crocodile.

Rusty

virus

Mother critter with young to protect.

Blue ringed octopus.....

or cone shell.....

Human was my first thought, but since you said “don't think of it as an animal, per se” I’ll go with virus

Mosquito

+1 for mosquito

There's a teeny tiny jellyfish in Australia that will kill you very fast. Not Portuguese man of war.....I can't recall the name.

Whatever the correct answer is, I bet it lives in Australia!

My first wife

I believe Hippos kill more people than any other mammal except humans. But then it's an animal so I guess that's not what your after based on your last comment.

I agree with human...life returns when humans leave...

It's probably some type of parasitic critter.

Black Widow - human or spider

Jim, you never disappoint!!!! :-)))))

Snakes

scorpion

Coronavirus