In real life magick works by apparently natural processes, but the job still gets done. As an example, here's what happened to someone who may have been the target of a curse. It's possible that this individual is lying rather than admitting to having some sort of mental illness, but under the right circumstances the Saturn evocation that I have posted here could prompt something similar.
The key is that you never specify the means by which a spell should work. If you want to injure someone, do you really care if they get struck down by a bolt of lightning or set themselves on fire? You shouldn't - magick is about results.
Suggested ebooks:Howard Phillips Lovecraft - The Curse Of Yig
Frater Hoor - A Thelemic Calendar
Paul Boyer - The Salem Witchcraft Papers Vol 3