Why does witchcraft work?

I have a theory based on the idea that god cast satan and the demons out of heaven and into the earth. If god and the devil exist, then wouldn't something like spells be possible through one or the other? Like if they have so much power then wouldn't it work with the use of worship? My point is witchcraft is performed using the earth, and according to some beliefs earth belongs to satan. I dont perform these or practice this stuff but I know people who do and one of them killed a girl with witchcraft. She performed the spell and the next day she died on a rollercoaster,thats just one example. This was someone I knew. I am only 16 so my ideas could sound dumb, but I just really wanna know how this is possible and get information on this.