User blog comment:MemoryAngel/The Story Of Light and Shadow/@comment-26921673-20180304032039

So in this hypothesis, the Devourer's venom did not turn Garmadon evil; it simply awakened something that was already there.

This makes sense, as Mistáke told of how "The evil of the Oni was waiting to unleashed in one of his sons- Lord Garmadon". This makes me think that it's more likely that the "evil" in Garmadon was "awakened" by the venom, not caused by it.