So, I'm thinking as being Olive and with her theory of being a serial killer, I wanted to know is there any interesting way to kill NPCs or currently unplayed sim?
It can be something absurd, and might not be a death option in game as it can be cheated that they died like that.
Also, is it safe to kill them, I assume I can, and of course not going to kill the ones who are dangerous NPCs, just to make sure I'm not going to ruin my game-

I had a Gardener die from hail while on the job. Didn't notice until her ghost came out and begun to re-enact her death They can also catch on fire, be fed to the cowplant, and even get struck by lightening. Flies may have an effect, too.

As for whether or not killing NPCs is safe, it is safe so I wouldn't worry. Service sims are fine to kill off. Just avoid the Social Worker, Repo-Man, and Carpool Drivers just to be on the safe side.

The cowplant is good for killing sims that don't live in households in the neighborhood.

There's poison cc that allows you to poison people- and the sims involved even get a memory of it.

And there's also Pandorasims extra witch spells- she could totally kill people by magic.

Weapons! Guns! Noisy, but they work!

I've been killing off a bunch of NPCs that the game was generating as discussed here using Rodney's Death Creator. They are "curing" on playable lots and will be moved to the new NPC crypt in the local cemetery.

The most terrifying/spectacular NPC death occurred when I placed Carmen Parker and the RDC tombstone on the fifth floor of an apartment house and chose death by drowning. The drowning animation caused poor Carmen to plummet through the floors of the building, flailing the entire time, until she vanished.