Lightning is one of the most dangerous forces in the natural world, according to the hex book. It says evil travels inside lightning, or something like that. Supposedly it can make you sick, which, I mean, yeah, I guess being struck by lightning would be pretty bad.
You have to understand, Granny uses the word “lightning” very broadly. My phone is lightning by her definition. She says lightning makes one more vulnerable to illness, and can make all sorts of terrible things happen. It should be avoided at all costs.
Lightning is a conduit for negative influences. It cleaves things in two, starts fires, leaves terrible scars on the land and the body. People touched by lightning are said to be cursed.