In "Twilight," Edward takes Bella to play baseball. Why does he tell her she shall be the death of him? I still don't understand. I think it's...

Bella and Edward are going to a field to play "Vampire Baseball," not soccer.  He is going to run through the forest because they can't drive.  He is intending to carry Bella on his back.  Before they leave for the clearing, Edward kisses her.  He has told her in the past not to move, but she can't help herself and grabs him around the neck.  As they kiss each of them becomes more aroused.  Edward tells Bella that she is going to be the death of him because he wants her so much he feels like he is going to die unless he takes her.  She replies,

"You're indestructible," I mumbled, trying to catch my breath. 

"I might have believed that before I met you. Now let's get out of here before I do something really stupid," he growled."

He knows that if he loses control he will begin to drink her blood and might not stop until she is dead.  The stress is strong for hinm and the quote he uses is a figure of speech to indicate to Bella how difficult she makes things for him when she gets so passionate.