The great mystery of evil

Evil is the greatest mystery in life; What exactly is it and where did it come from?

According to the Bible evil came into mankind through the original sin. So the devil's the personification on evil but how did evil entered him who used to be the most glorious of angels?

But one thing's for sure, evil cannot be meaningful unless goodness exists. At least christianity offers a framework where God is the personification of goodness and his purpose good.