Native English speakers PLEASE help.
CORRECT ME.
--- you shouldn't marry with an old man, are you crazy? He's got wife.
but john loves you. he is young.
{why are you killing the goose that lays the golden egg.(this expression is really important to me)}

Question

Native English speakers PLEASE help.
CORRECT ME.
--- you shouldn't marry with an old man, are you crazy? He's got wife. 
  but john loves you. he is young.  
{why are you killing the goose that lays the golden egg.(this expression is really important to me)}

Answer

http://idioms.thefreedictionary.com/kill+the+goose+that+lays+the+golden+egg

You kill the goose to find out where the golden eggs come from and then you have no more eggs.
The context doesn't really make the meaning of the idiom clear in your story.

Answer

Assuming 'he' is John, it looks good to me! 
"You shouldn't (can't would be a better word) marry him, are you crazy? He's got a wife, but John loves you...

Answer

You don't "marry with" someone, you "marry" someone.
So, it should be:
"You shouldn't marry an old man, are you crazy? He's got a wife. John loves you. He is young."

Also, "killing the goose that laid the golden egg" doesn't make sense in this situation.