Man cannot know evil without becoming contaminated by it.
Because his knowledge is of the aspect of the inner Light, it becomes part of his essence, what he learns becomes part of his very self.
Who can say with utmost sincerity that he has met something that leaves absolutely no impression on him?
Whatever a person gets to know changes him.
Unlike a computer, which can contain lots of information and erase it entirely from memory and from itself, as though nothing had ever been there, a person whose consciousness absorbs something, no matter how trifling a sight or a sound, can never obliterate it as though it had never happened.
This is in contrast to the reaction in the king's palace to the entry of a house-lizard.
Odious as the creature may be, it cannot be totally kept out but it has no effect whatsoever on the palace.
In the aspect of the encompassing knowledge, evil can be just like a house-lizard in a king's palace.
It is a knowledge of something outside one's actual existence.
And there is no interaction with it.
Thus, after committing what has been called the sin of eating the fruit of the Tree of Knowledge of Good and Evil, man is no longer the same.
He has to go off on another way, one of struggle and suffering and death.
The innocence cannot be recovered and "meaning" has to be given to life.
The angel has no such obligation imposed on him; he can know good and evil and remain an angel.
When man gets to know good and evil, he can only with difficulty remain a man.
–Rabbi Adin Steinsaltz
From In the Beginning by Rabbi Adin Steinsaltz