I absolutely think that "Professor" Snape is a bad guy in this book. Don't you remember at the beginning Snape said it was all his plan to pretend to be on Dumbledore's side. It is hard to think that Dumbledore was really fooled be Snape, although I really don't think that he was. I think that Dumbledore knew, somewhat, all along, but he wanted to keep Harry from finding out his speculations and doing something really foolish. Also I don't think that Dumbledore was pleading for his life I think that he was pegging Snape not to do it because that would mean he was on Voldemort's side and that there would be no way for him to return back to the good side if things got to hard.