Might be the wrong forum for this but oh well, bombs away.
Knowledge of Good and Evil makes you god like.
Gen 3
4 "You will not surely die," the serpent said to the woman.
5 "For God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil."
23 "And the LORD God said, "The man has now become like one of us, knowing good and evil. He must not be allowed to reach out his hand and take also from the tree of life and eat, and live forever."
It's because people think they are as good as God. Basically because God says so, also see above.
The serpent is most likely just a literary device to give Eve a motivation to eat the apple, unless you believe in a literal reading. The bible gives no indication that she wanted to be like God, just that she wanted to be smarter and the fruit looked yummy.
6 When the woman saw that the fruit of the tree was good for food and pleasing to the eye, and also desirable for gaining wisdom, she took some and ate it. She also gave some to her husband, who was with her, and he ate it.
In a nutshell it goes like this.
Snake "Hey Eve, why don't you eat the fruit?"
Eve "God said not to."
Snake "So?"
Eve "Duh... ok!"
Eternal damnation and suffering incoming.