I've got just a couple small ideas for them to spawn around the world, without killing npcs or yourself.
1.) Over time, they could randomly spawn on the surface and maybe in the Caverns, Underground, and even the Underworld would kinda make sense.
2.) Singular houses or maybe some small, abandoned towns have a chance of generating above ground and maybe also underground, with a small cemetery with 3-8 gravestones in them. The town would be 1-3 small houses, and the underground one would be underground houses specifically, just all around each other and clustered together.
I won't lie, I didn't know NPCs could drop em before making this post (ya boi sticks to just master mode and that's it really, so I went to the wiki just to make sure and boom, now I know), but I still wanted to share these other ideas that I had. I did also have the npc idea, but I thought adding a teeny bit more detail to underground/above ground structures could be awesome.
Image is from this other thread
I hope that other thread links properly, I just got it off Google if I'm being real.
by IceTutuola