Of what does Huck believe his accidental killing of a spider is a sign?

Huck believes the spider is a sign of bad luck.

I didn’t need anybody to tell me that that was an awful bad sign and would fetch me some bad luck, so I was scared and most shook the clothes off of me. I got up and turned around in my tracks three times and crossed my breast every time; and then I tied up a little lock of my hair with a thread to keep witches away.