Ummm... what the heck was the dog doing loose? ...and at the airport too?  If owner can't control the dog, who else do you expect can?  Luckily it was a security guard and not a child walking by.  Sorry... if I had a gun like the security guard, and I saw the dog going after my toddler... I'd do the same without hesitation.  If dog was on leash... this wouldn't have occurred.  Lesson for owner to learn.