Killing Spiders is bad luck.

My informant first heard this from his girlfriend a few years ago. He described his girlfriend as being “really superstitious,” and seemed to disregard most of her other beliefs. Perhaps his belief in this is related to his arachnophobia.

This folk belief can also be found in The Adventures of HuckleBerry Finn

Twain, Mark. The Adventures of HuckleBerry Finn. p 171

http://books.google.com/books?id=n2FA-nwz7GYC&lpg=PA171&ots=Zpmm96UjrP&dq=tom%20sawyer%20spider%20kill&pg=PA171#v=onepage&q&f=false