In the Old Testament he actually isn't as evil as people perceive him to be. Basically he tests humans. Hence the saying Devils Advocate.
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This never made sense to me, why would an omnipotent being who knows the past and future and all outcomes need to TEST humans? That doesn't make any sense. You test something to see what the outcome will be because you don't know, but when you already know the EXACT outcome then there is absolutely no point.