Knowing something or somebody isn't the same as knowing about them. More than just information is involved. When you are a knower, you don't simply add to your mental store and go your way otherwise unchanged. To know is to participate in, to become imbued with, for better or worse to be affected by. When you really know a person or a language or a job, the knowledge becomes part of who you are. It gets into the bloodstream. That is presumably why the Tree of the Knowledge of Good and Evil was the one tree Adam and Eve were warned to steer clear of.
When in their innocence they knew only good, they could be only good. As soon as they knew evil too, a whole new glittering vista opened up before them. Next to obedience appeared the possibility of disobedience; next to faithfulness, faithlessness; next to love, lust; next to kindness, cruelty; and so on. Even when they chose the good way, their knowledge of the evil way remained as a conscious and by no means unattractive alternative, preventing them except on the rarest occasions from being good wholeheartedly. And when they chose the evil way, their knowledge of good tended to turn even the sweetness of forbidden fruit to ashes in their mouths. Thus they became the hapless hybrids their descendants have been ever since. It was the curse God had tried to spare them. The serpent did its work well.
According to Thomas Aquinas, God can know evil by pure intelligence without becoming tainted by it the way a doctor can know the nature of disease without becoming diseased. Humans, on the other hand, not being pure intelligences but creatures of flesh and blood inhabiting a world of space and time, can know only through the likes of experience, experiment, will, and imagination, and once they start knowing evil that way, the fat is in the fire.
~originally published in Whistling in the Dark and later in Beyond Words