Killing spiders is bad luck.
My informant says that when he finds spiders in the house he takes them and relocates them outside instead of killing them. He learned this folk belief from a nun in grade school. The nun didn’t them why it was bad luck but he rationalized it as if you relocate the spiders you’ll get fewer spiders. He figured that if you smash them they’ll multiply and come after you. He also states them relocating them isn’t necessary; you just have to avoid killing them. The reason he relocates them is because the people he lives with are afraid of spiders. He doesn’t still believe that spiders will come after him but he still relocates them anyway.
I’ve heard this folk belief before but I never associated it with Catholic nuns. The version I’ve heard focused on the fact that spiders kill pests so it is not a good idea to kill them. Maybe this belief is related to a “you should value life in general” dimension to this belief. The “killing insects” is bad luck folk belief is something I’ve heard before but generally the one I was taught was that its bad luck to kill ladybugs. I understand why killing either would be bad luck, ladybugs and spiders kill pests so their good to have around. In my house whenever someone sees a spider they want it dead. Maybe its because ladybugs are rarer or maybe its because spiders aren’t cute. For some reason spiders aren’t generally given the same amount of reverence other bugs get.