Best friend should always be written as two words, not one.
An ellipsis has (say it with me) three dots, not two. An ellipsis doesn’t quite seem right in this example, but if you must use one, use a real one, ‘kay? I recommend replacing the ellipsis with a comma unless you have a reason for a long, dramatic pause there.
A comma after her is necessary because, although quite short, the sentence is compound.
She is my best friend. Hurt her, and you’ll die.