"Is it not possible that when he is sober he is real,and he is a good person"
Yes, he likely is a "good person" when he is sober. But he is not sober and the way he is treating you is not good.
"If he had Cancer I would be a bad person if I turned my back."
Not true. First, heis NOT your child and there is no moral obligation of you to stay with him because he has the disease of alcoholism. Second, Alcoholism is not anything like cancer. He has a choice to drink or not. He chooses to drink. People with cancer have no choice.
"So, why is this a disease, If the way I handled it is to just give up?"
You do not need to decide this right now. If you really want to understand the disease nature of alcoholism, you should get a book that explajns it. I did and I learned a lot.