Stupidity: John, the guy who taught Chucky the voodoo spell is confronted by Chucky later on in doll form. When things don't go how Chucky wants, he finds a Voodoo doll of the same guy just lying around and uses it to torture him. So this idiot who knows Voodoo and what it does, happens to have a Voodoo doll of himself just lying around? Yeah sure, that can't go wrong at all.
Quantom X
Chosen answer: He taught him how to call forth and harness magic. There is white magic and black magic. It's the individual's decision on which to use.