I'm so sorry for your loss. Buddy was an adorable little guy, and I know you must be heartbroken. There is no excuse for Petsmart's behavior!
I can't believe it was the manager's dog, but more than that I can't believe they would allow dogs that have not been temperament tested by a professional (much like they do at doggie daycares) to hang out together, especially with such a large size difference. Rest in peace Buddy.